I guess that even if you subtracted every keepers two worst games, De Gea's stats would still compare very favourably.
That was an interesting one, well thought of. So it turns out that if you knock off the two worst games from all keepers in the league, the worst keepers benefit the most. Which makes sense once you think about. If you look at Mark Travers for example, he had a whopping -5.1 PSxg-GA playing for Bournemouth in the 9-0 against Liverpool. Obviously, if you take that away, his overall stats improve a lot.
The top 6 keepers in the league (ie the only ones with positive PSxG-GA) all stay in the top 6, in mostly the same order. However further down the rankings, some players start to jump around.
What it seems is that there are two types of keepers in the middle of the pack. Some who are consistently bang average, like Leno, and some who are boom or bust, like Henderson.
Henderson has managed the 2nd and 6th best goalkeeping performances this season. He's also managed the 6th, 9th, 10th and 11th worst performances of the season. (A quick glance at the Henderson thread kind of backs this up - people alternating between praise and derision)
De Gea's performance against City is actually the 2nd worst out of all 290 odd performances this season. The performance against Brentford was the 12th worst. Fun fact - Henderson and De Gea make up 6 of the 12 worst goalkeeping performances in the league all season. Lucky us! De Gea hasn't had the same standout performances as Henderson, but has about 5 in the top 20%. This speaks to the inconsistency.
So just based on PSxG-GA, De Gea is ranked 16th. Take those two worst performances away and he jumps up to 7th. That's the biggest change of any keeper. The keeper who jumps up the second most is...Henderson. (I feel like those two are tied by threads of fate).
tldr - yes De Geas stats do look a lot better if you ignore the two worst performances, but the fact they change so much is a sign of a problem. Good keepers are consistently good, and their stats don't change much when you knock off a couple of games.