Quality of shots, teams they play for, all factors, I mean imagine Petr Cechs stats for that Jose team.100% agree I do think its hard to rank keepers, whether its correct or not I don't know but have seen it mentioned about things like post shot expected goals/90 mins, stuff like that (think he ranks at 43 for that) I did a quick google and it comes back with that also, a few other things about save %. I totally agree and think judging keepers is tricky can depend on a few factors imo.