"finished in top x% of your category" on race /events screen doesn't make sense to me

I did one of the Dempsey challenges a few weeks ago, and the result screen pops up saying I finished in the top 15 % of my category. Just above that, it indicates there were 70 starters in my group and I finished 35th. So how does zwift come up with 15%? Just curious, thanks

it works on your FTP and category ie A B C D

Thanks Paul.