It's not enough to just put a tiara on your head. Any old fool could do that. If you're a real Royal woman, you'll wear your tiara at precisely the right angle. Once upon a time, the done thing was to wear a tiara close to the front of the head, but things have changed in more recent years.
Both Catherine and Meghan have probably had style advice (and maybe even lessons in geometry) from royal etiquette experts on how to angle their tiara at precisely 45 degrees, and how close to the back of their head they should wear the expensive headgear. If you want to try the style for yourself at home, it should run across your head from a level that's just behind your ears. The previous fashion for wearing it further forward dates back to the 1920s but has proven difficult to maintain because of changes in hairstyle trends for women.