Joe Little, managing editor Majesty magazine, tells PEOPLE the cause behind the curtsy is simple.
"It is rare, but it indicates that this was the first time that the Duchess saw the Queen," he says. "It would normally happen in private. Although they came from Sandringham House, they obviously hadn't seen each other before Kate and the Middletons and their friends set out on foot."
He adds, "It's unusual to see it in public but not unique. That would have been the first communication between them on that day."
It's thought Kate and her husband Prince William, 33, initially came from their own home, Anmer Hall, and met up at the Queen's grand Sandringham house. They then strolled to the historic St. Mary Magdalene Church, where Kate, who celebrated her 34th birthday on Saturday, met with the Queen, 89, and Prince Philip, 94.
There is a common misconception that women in the royal family curtsy to each other, Little adds: "The only woman the women in the family curtsy to in the royal family is the Queen."
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Kate was joined by close friends Sophie Carter and James Meade (who are godparents to 8-month-old Princes Charlotte), and Meade's wife Lady Laura, along with one of 2-year-old Prince George's godparents, Oli Baker, and his wife Mel. (Both old friends of the couple from St. Andrews University.)
As she left the church service, Kate told well-wishers that George had done well at his first day of preschool on January 6.
She and William spoke with Arthur Coxon, 88, a retired Royal Navy lieutenant commander whose father fought in Gallipoli, following a ceremony to mark the 100th-anniversary of the final withdrawal of troops from the Turkish battleground.
"I congratulated Kate on the lovely photos of Prince George," Coxon told reporters. "She said he went off to school bravely as anything and thoroughly enjoyed it."