Skill level changes all the time.
In hindsight, o.hawk being top tier is obvious. People just learned game mechanics properly.
Lots of times some countries won't play the game the same as everybody else and then they're thunderstruck when someone comes in and starts playing it properly. 3S in America was really, really, really stratified, with everyone obsessing over top or high tier characters and nothing else. Meanwhile in Japan they played almost every character in tournaments. The Japanese acknowledged that some characters were undeniably better, but took the time to learn the entire game instead of just six characters. This meant they could beat the best American players playing the best characters, sometimes with top tier and sometimes without.
Also tier lists actually don't change that much. If someone moves up or down four tiers or something and it isn't a situation like o.hawk, then we're probably talking about decades of meta development. Even then it rarely happens.