I have been thinking a long time about it and I concluded it is about one being methodical.
A methodic person tends to put extra attention on (all) the tasks they perform, as well as they are more critic to correctness, and those features combined might make them “better programmers”.
People who take the time to write their code carefully understand that they're writing the code for other people to understand, not just for a machine. They're considerate of others, not wanting them to waste their time in the future trying to make sense of sloppy code. The same is true for e-mails, doc and other written communication - if I take an extra minute to make my message clear, it can save a lot of confusion down the road. (This is even true for things like Slack messages and HN comments.)
Beside, one would think that proper English (which?) grammar and orthography comes easier to native speakers. The same isn't true for Math and logically stringent thinking, which I'd think is more closely related to programming.
Regarding the use of the apostrophe, I don't have any proof but I think it's more often native speakers rather than non-native speakers that make theses mistakes. Especially it's vs its. I would be really curious to have stats on this and more generally on the categories of errors that non-native speakers make compared to native speakers.
I think it's because a non-native speaker has to learn the grammar in a systematic way, as opposed to a more organic/phonetic way. For example we (non-native) learn "it is" before learning "it's", which I imagine is the opposite of a native speaker.