Plus a lot of people have tangential links which, depending on how close lipped people are, can result in little hints. I can think of three I had off the top of my head which could have produced information
I knew the wife of a close protection officer, can't engender if that's the correct title for the police who do that, at Anmer Hall.
A relative was church warden at a church the Queen used to visit often and knew the people she visited.
I was friends with someone whose parents worked for the Royal family.
I don't have any inside knowledge at all but if those people had been less discreet I'd have gossip to spread. I'm really no one at all so if I have 3 contacts I can see how people better placed would know loads. It's six degrees of separation come to life.