Do you think bull terriers are "mutant" dogs?

I have heard people say this and I just want to know what others think and WHY? I actually own this breed and they are fine in every way. They don't have any issues doing anything. They have a lot of energy and can breath/run/play just fine. They are great at various sports and I know of some that are titled in weight pull, agility, schutzhund, and even tracking (I am sure there are more). I know they have changed a little from when they were first created, but pretty much all breeds have. So how are moder day bull terriers mutants?