You're right in saying there're many factors that can influence something like a save percentage. The WHL, for instance, is recognized as more of a defensive league, so that could be a factor, for example. Consider the OHL though, which by and large produces far more than the Q, by way of offensive players.
Looking at the first few games this season isn't as telling as the last 5 seasons as a data set (which I took 2 or 3 minutes to do).
The Mem Cup, again, isn't a particularly compelling set of data. No doubt, that all top teams likely have good goaltending. Looking at 60 teams, as opposed to 4, is more telling.
Maybe I could've used a word like "fair", or "notable" as opposed to "considerable". Mere semantics, I suppose.
Anyway, I've no doubt there are many variables (like the ones you mentioned) affecting this stat. There's of course more than just that stat. Still though, I invite you to expand your analysis beyond a handful of games and an annual four team tournament.