Maybe it's the fact that they're tiny, stylish look-alikes with matching white hair.
Strangers often stop Jeanne Slatter Strickland and her identical twin, Lois Slatter Dye, on the street. Sometimes, they ask to take photographs and sometimes, they take pictures without bothering to ask.
"That's just happened since we've gotten older," said Strickland. "We've even had people stop and jump out of their cars to take pictures, right here in the street."
"People on the street stare at us, especially as we've gotten older," said Dye. "Or they point."
"They don't usually see twins around who are so old," said Strickland.
The Slatter twins born in Oakland and raised in Davis turned 90 earlier this month. Both widowed, they're independent and sharp, the pride of their extended family, which is happier than Strickland and Dye with the occasional celebrity of their twindom.
"Aren't they a kick?" said Strickland's son, Craig Strickland, 56, who teaches elementary school in Carmichael.
The Slatter twins don't think so. They're modest, a little embarrassed when attention is drawn to them, and they really don't like being called unusual, even though they are.
In the world of their birth, twins were a relative rarity, an unanticipated surprise. In 1922, the chance of a woman having twins was one in 90 and only one-third of those twins were identical, resulting from the splitting of a single fertilized egg into two embryos, said Dr. Bill Gilbert, an obstetrician and founder of Sutter Health's Moms of Multiples Center.
In contrast, the twin landscape has changed drastically in recent decades, primarily because of the widespread use of fertility treatments and women's delayed childbearing.
The rate of twins has soared by 76 percent since 1980, according to the federal Centers for Disease Control, with 33.2 births per 1,000, or about one in 30. For mothers in their late 30s, the twin rate has doubled and for mothers 40 and older, it has tripled.
In 2009, the National Vital Statistics Report shows there were 143,557 births of multiples in this country. That number included 6,340 triplets and other higher- order births.
Multiple births today are easily anticipated but not in the 1920s.
"Now, most twins are diagnosed by ultrasound," said Gilbert. "But before ultrasound, women found out they were having twins during childbirth."
As Jeanne Strickland said: "Everybody was surprised when we were born, the whole family. They got two at once, and we were the only girls."
They were named Jeanne and June before their parents thought better of it and changed June's name to Lois. They weighed more than 5 pounds each at birth, and they had two older brothers and, later, a younger sister.
Their father ran a soda fountain in Oakland before moving his young family in the late 1920s to Davis, where he built Slatter's Court, a complex of tourist cottages on Olive Drive near the university that today long out of family hands also includes a mobile home park.
"Our parents wanted to raise their children in a small town," said Strickland.
"It was very safe in Davis then, believe me," said Dye.
After they graduated from Davis High School in 1940, the twins attended then-Sacramento Junior College, leaving when war broke out and classes were curtailed.
Lois Slatter worked briefly for the telephone company before marrying Eugene Dye in 1942. He served in the Army in the Pacific, then returned home to work as an executive in the dairy industry. They had three children and lived throughout California and in Venezuela, finally settling in the Bay Area.
They returned to Sacramento in 1984, after Eugene retired. He died in 1999.
Jeanne Slatter worked for a bank in downtown Sacramento until 1943. She married Winson Strickland, a young Texan she met in Davis, where he was in the U.S. Army Signal Corps. After his service in World War II and Korea, they lived in River Park. He worked as a banker, and they had three children.
In 1968, the family transferred to the Bay Area, and Winson died a few months later of an aneurysm at age 52.
"I lived at home until I married, and then my husband took care of me," said Strickland. "When he died, it was kind of a shock. I knew I had to get out and get a job that would provide a retirement. It all worked out."
She returned to Sacramento in the early 1970s and went to work in accounting for Sacramento State's Hornet Foundation until her retirement.
That's when the twins decided to travel together, along with Lois Dye's husband, and they made a startling discovery.
"Our mother always said that Lois was born first, and for 50 years, we thought that was true," said Strickland.
"And then we started traveling and needed our birth certificates," said Dye.
"For the passports," said Strickland. "And on the birth certificates, I was No. 1. I'm the oldest by 10 minutes. I could really be Lois, and she could be Jeanne."
The two of them beam with delight at the thought, sitting together in Strickland's tidy home near Sacramento State, a block from Dye's residence.
They walk together every morning and go to exercise class together twice each week.
When they go out together, they check in with each other ahead of time to make sure they're not wearing the same color. Their mother dressed them alike for the first 13 years of their lives, and they're over it.
Every evening, they have dinner together, one night in Strickland's home, the next in Dye's.
For their birthday celebration, their children gathered the generations for an afternoon reception followed by dinner.
"They sat at the head of the table at dinner and had four generations there with them," said Craig Strickland. "The cousins and I always say we're lucky they have each other."
In the twins' younger years, before modern medicine lowered the childhood mortality rate, they survived the infectious diseases that proved fatal for too many. They weathered the Depression and, much later in life, they have made their way in good shape through the potential health minefields of age.
They have outlived their spouses and their siblings.
And now they're white-haired identical twins in a world that's accustomed to much younger multiples.
"People come up and say, 'I have a twin grandchild,' " said Strickland.
"Or 'I'm a twin,' " said Dye.
"They just want to let you know, I guess," said Strickland.