I'd like to comment that we have often seen photos of the Duchess before she hits a public moment. They all look like this. She takes a moment to gather herself. The Duke oCa does this as well. In fact, most of the family walks out to these garden parties calmed and silent rather than smiley and waving at the crowds or chatting among themselves. We have even seen William and Catherine at joint appearances where they are standing next to one another, silent and looking in different directions. They are getting ready to go "on" as their public personas.
I don't want to turn this into a discussion of the York girls - but find it absolutely normal, given their different lives, that these cousins in law are not besties.
Sometimes, I just wish the DM would grow up!