Keep in mind also that there's likely a little bit of beginner's luck going on here (not a bad thing as the team has to milk that as well).
Not every team has faced Koskinen and the book may not completely be out on him yet to fully expose his weaknesses and openings. Sure teams make big use of video... but it's never quite the same as playing a team and facing a goalie 1st hand.
While Koskinen is still somewhat of an unknown commodity... I think playing him heavily is a solid tactic as the team gets that slight exploitable edge vs opponents and the book is much more clearly out on Talbot... for better or worse his flaws and openings are likely more well studied and known about as teams have faced him many more times.
The thing with Koskinen though is that he's much better athletically than Talbot.
Talbot relies much more on positioning, so when teams scout how he positions himself and his weaknesses (glove side, side to side movement across the crease), he becomes very easy to beat.
Koskinen has a similar weakness (glove side), isn't as well positioned as Talbot, and has slightly weaker rebound control than Talbot, but his athleticism is leaps and bounds better. He also seems to be stronger mentally at this point.
Even once teams scout his weaknesses, his athleticism should allow him to continue to make more saves than Talbot would. His ability to move on after weak goals/games is also important.