The assumption that people who make these mistakes do it because they do not understand the grammar is very wrong. It's a frequent mistake because the words all sound the same and so when typing quickly its easy in your head to write the wrong one down. The writer will usually notice it and decide whether or not it is worth fixing. If it is in a casual scenario there's really no point in fixing it as you'll be understood regardless. It's basically just because of laziness, we fully understand it from a grammatical viewpoint, it just slips up because they sound the same. A lot of language is just knowing what sounds "right". That doesn't work so well when many words sound the same. You'll almost never hear an native speaker mess up using "a" and "an" because it sounds SO wrong and if they do they'll usually correct themselves.

This isn't an "English native" thing. This happens with every language. Native speakers are always more sloppy as they have a deeper understanding of the language and know when they can and cannot be understood in terms of making mistakes. Native speakers don't have to "learn" their language they just pick it up as they age. We don't learn rules like you do when you learn another language.