There is no evidence that that is why they have lowered the offer for you. I am at Exeter for physics, and their usual offer was around AAA, but I was offered ABC. I went to uni only to find out I was the only person given this offer.
I think that sometimes universities will lower your offer if they really want you and see a lot of potential. Sometimes, they'll lower a normal offer if the school you went to is on a standardised list of struggling schools (like mine was).
Moral of the story, if Kings is where you've wanted to go, and you know why you want to go there, then your surprisingly low offer should not detract you. It is a very good Russel Group uni, and a good grade from there will be worth a lot!