The term overachiever is usually seen as a positive descriptor. By definition, an overachiever is someone who achieves success over and above the standard or expected level. But right there is the key — over the expected level. In hockey, more often than not overachievement leads to a letdown just one year later.
Luck plays a pretty sizable role in hockey. Goals win you games, and the ratio of goals for to goals against with any given player on the ice can be quite volatile year to year. The lack of repeatability, especially in extreme cases, is why things like Corsi and Expected Goals are even desired in the first place.