Calibration and environment.
Not overtly useful for goaltenders, doesn't take influential environmental factors into account. Examples: Ondrej Pavelec, terrific this year for Winnipeg in a desolate waste-land. Game stealer when he was on. Certainly some bad games now and again, but not enough for him to be reasonably considered on the list. On the "best 20" side, Jaroslav Halak and Brian Elliott both top-13, systemic products (note: Elliott was bottom-5 last year when on a poor defensive team). Mike Smith rated #1 player in the league, but no context re: Dave Tippett and counterparts (Jason LaBarbera).
Especially regarding goalies, it's merely a stat-grab (save pct.) and nothing else. Doesn't have much (if any) value in terms of player evaluation.