I think it's very rare in general that a keeper will be able to come out and claim every cross perfectly. This season I have seen Krul/Vorm/Reina/Hart/De Gea/Cech all flap at a cross to two, so I think that's a rather mute point when comparing any of these keepers, as it is neither a regular party of any of their games, and nor is it such an issue that many goals have been score from these 'mistakes'.
But its still an error, when you come for a ball that you dont need to come for, and miss it completely. Its these kind of errors that are held against De Gea, even when they dont lead to goals. A mistake is a mistake, even if it goes unpunished. Though he has made less than Reina for sure