Until you look deeper into that run of games. At Chelsea we defended deep which is what suits De Gea's game best because the it means the defenders are better placed for crosses and there simply aren't the balls in behind to deal with, allowing him to focus on the one thing he's good at (which I don't seem to remember he had to do a lot of being it was a dreadful game). De Gea was at fault for two goals in the Everton game, should have collected the crosses West Brom and Newcastle scored from, was in no mans land from a corner for Sheffield United's first goal, etc. He had been in abysmal form leading up to his spell out of the team.
I'm not sure why you've only looked at Premier League games but contrast the 8 games on the bounce Henderson played with the previous 8 games De Gea played. 4 goals conceded vs 7. 6 wins (inc City & Spurs away by the way) vs 3. The statistics across all competitions were even more blatant.
We played a defensive game against City too when we won 2-0 with Henderson in goal. We had 34% possession and conceded 23 shots. I don't really remember us ever playing with high defensive line under Ole.
I'm not defending De Gea, he was godawful in that season actually. Just don't think the defense wasn't that bad statistically except for that Everton game. 3 out of those 7 goals came in that one game, but the rest were fine.
De Gea was good last season in my opinion though.