The authenticity thing is very simple: if one is to compare William and Kate, no pretense is required. They are actually next in line for the throne, so they’re free to be whoever and whatever in whatever space is permitted around that. Charles wasn’t ever interested in “common” stuff, but seems William always was (probably his mum’s influence - Thorpe Park, Macdonald’s etc) and Kate was brought up as a “commoner” (albeit a fancy one) so it literally is what she is.
Harry still has some authenticity in that he doesn’t seem to care for clothes and fast cars and palaces. At least, he didn’t until Meghan came along. (Still doesn’t seem like he cares much about clothes!). Yes he’s interested in polo, but he also loved rugby, pints in the pub, roast dinners etc. Basically, he’s old money so all this “normal” stuff is blessed relief - because he always had the knowledge his real place was immeasurable luxury. Now that he’s lost that, I expect he’s a bit lost on this front. I imagine anything nouveau prob makes him grimace; but he’s having to reckon with such things as bills and debts now. Who knows where he’ll end up. And in California of all places. (Not for the first time, what was he thinking?!)
Meghan is new money through and through. She’s not even good at pretending she’s not, hence the inauthenticity. On the one hand she’ll tell you she was an actress and feeds her kids chicken nuggets and loves fast food; on the other she won’t miss an opportunity to show off her expensive clothes and house and general lifestyle. She can’t help it. It pulls the rug out from the fast food and chicken nuggets because it’s clear where she really wants to be, and how deeply she cares about the money.
(Before anyone come at me, I’m very aware how snobby all of this sounds. I was raised in a world probably not unlike Kate’s, easy to spot the Meghans at a hundred paces. I would like to think I’m not a snob myself because, having been raised in a world not unlike Kate’s, I also know how cringe and besides the point actual snobbery is, and the value and pitfalls of money…).