There aren't many positions across professional sports that see greater volatility in performance than that of an NHL goaltender.
It's not uncommon in the slightest for goalies to follow up a relatively strong season with an immensely disappointing season, only to turn it around the season after that, reaching or exceeding their previous levels.
The inverse is true, as well. It seems as though there are a few goalies every season who come out of nowhere after everyone's written them off as the league's backup fodder.
There are plenty of contextual factors in play here that contribute to those swings, but it remains true that individual goaltending performance is incredibly inconsistent.