Prince William technically broke royal protocol by not bowing to Queen Camilla during the Coronation.
A body language expert believes the Prince of Wales may have found it unnatural to perform such a gesture to his stepmother.
Instead, Judi James said his gut instinct could have seen him subconsciously side with his late mum, Princess Diana.
The bulk of the Royal Family had already taken their seats for the special concert at Windsor on May 6 when King Charles and Queen Camilla arrived.
Most of the Firm then bowed or curtseyed to the King only.
Prince George and Princess Charlotte even took their father's lead and failed to formally greet their grandmother.
However, their mum Kate chose to make a point of curtseying to both.
Ms James told the Express.co.uk that royal protocol states lower royals must perform a bow or curtsey to those in a higher rank.
And with Camilla now above the Prince of Wales, he should have done as such.
However, she said there is a possibility that he had already done so earlier in the day - off camera - and didn't feel the need to do it again.
Ms James said: "Would William have felt comfortable bowing to the woman who took the place of his mother on the throne of the UK?
"He turns to look at his father but avoids looking at his stepmother."
In contrast, she said Kate offered a "warm" gesture to her mother-in-law.
"This is a unique example of selective bowing, where William, Charlotte and George opt to not bow or curtsy to Camilla in keeping with protocol, but Kate drops a deep curtsy to both Charles and Camilla," she said.
"It looks like an appeasement gesture after Camilla has been elevated to the role of Queen."
Ms James opined: "Kate engages Camilla in some eye contact and a warm smile before dropping into a curtsy that she might not have performed before the Coronation.
"There must have been a sense of awareness of Camilla’s new role and status and we can see the Duke of Gloucester dropping his head before Kate makes a similar gesture.
"Kate smiles at Camilla here before curtsying and it appears that she is still in the role of royal peace-maker here."
The body language expert argued this was likely because Kate will one day be a Queen Consort and would want the same respect afforded to her.
Ms James added: "Meanwhile, Kate looks keen to register an end to any sense of animosity or resentment with this one simple gesture of respect from a future Queen to the current one."
