Judi James told The Express how Kate and Her Majesty sharing a kiss at the May event was a show of their friendship.
"Kate was always a slow but steady integrator into the royal firm and her slightly traditional approach, making very few if any ripples in continuity, does seem to have resonated with the Queen, who increasingly appears to treat her as a friend as well as a daughter-in-law," she explained to the publication.
"Like the Queen, Kate is an adept 'masker', using body language techniques that appear to keep her inner feelings hidden," James added.
"Both women have perfected a social smile and non-verbal traits that ideal for the job they do but without encouraging any frenzied interest from the public. You rarely if ever see either woman look anything other than pitch-perfect so we assume their lives are, too."
But their toothy grins are 100 percent authentic, James said.
"The Queen’s sudden broad and authentic-looking smile said it all at the Chelsea Flower Show this week: it appeared from nowhere as she caught sight of Kate approaching, registering instant warmth for the future queen, while Kate’s part-smile part-laugh response showed her genuine excitement at hosting her grandmother-in-law.
As for Prince Harry's wife Meghan Markle, according to James, The Queen is still in the "forming stage."
"Although Kate and Meghan are the same age, the newness of Meghan’s integration into the royal firm means the Queen appeared to treat her as a younger royal.
WATCH: KATE SHOWS OFF HER GARDEN TO QUEEN ELIZABETH