I remember there was evidence that they rated Dark Knight has way more difficulty than other tanks at some point. It may have been Shadowbringers? And obviously it was one of the easiest tanks in terms of rotation. It just didn't feel that way for the first 5 minutes or day of playing. Once you're used to it, it's not very difficult at all, but I doubt the developers play it for more than 5 minutes. Hence the difficulty rating.
They should just scrap difficulty ratings because difficulty depends on the person and how much time they play it for.