Washington is a good defensive team - they are better than league-average at suppressing opponent shots across the board:
But they have not been better than average in goal:
As others have noted, special teams save % is much less ability-driven than even-strength save %, no doubt because teams spend about 48 minutes per game at even-strength compared to roughly six down a man, and 0.2 down two men.
Washington may have in Semyon Varlamov and Michal Neuvirth a pair of goaltenders who add up to league-average if they split time 50/50. But Varlamov has a nagging injury that limits his availability and resulted in the Caps giving 1/8 of their playing time so far to a goalie who's not ready for prime time. You can't pretend that didn't happen.
EDIT: Let's run through Washington's goaltending scenario:
Varlamov - career .930 SV% @ even-strength. Likely higher than his true talent (few goalies have talent this high) but let's credit him with it anyways.
Neuvirth - career .916 SV% @ ES. Possibly true talent.
Neuvirth starts 45 games; Varlamov gets periodically injured, but starts 30 games when he's healthy; and the remaining 7 games are filled by replacement level (.890.) What do we get?
A .919 even-strength save percentage, which is below the league-average. It doesn't matter that a 100% healthy Varlamov would be above-average.