The limit is 6 characters, not 10. They apply on all sites in the network, but only to suggested edits; on Mathematica Stack Exchange, where you have 11k reputation, you can freely edit posts. (Tag wikis and excerpts are also exempt from the limit.)
For an explanation for the reason of the limit, I'm quoting an old post by staff member Grace Note:
Remember, when you suggest an edit, it requires multiple other people to look at it and approve it. The character limit is to prevent people from wasting time by looking at exceptionally minor edits. So, don't limit yourself to just a tiny edit: try to see if you can improve the post to a possible state of perfection. If you hit all errors on a post, then no one else will even need to edit it.
If there's absolutely nothing else you can improve to the post, leave a comment so that the author (or another passer-by with enough reputation) can apply the fix.
- The original reading is confusing or misleading, but the author's intention becomes clear after a simple substitution (e.g. "not" -> "note")
There's no way for the system to determine whether that is the case or not. The second one might be because those words start with a capital letter; on the other hand, those edits aren't as urgent as the first case and a comment works fine in those cases.