I'm guessing it's like with cars, where different sizes, breeds etc incur a different price? As I'm paying more that (by a wide margin) for my 3 year old in the same category. But he is 16hh+. And a WB.
Maybe WBs and their "special" nature makes them like owning a ferrari haha!
I pay £35 a month for my percheron yearling with petplan under category 2 (due to being a heavy horse) I paid more for petplan as I never had any issues with them with our other horse.