Most healthy kids. Kids who grow up in supportive homes. But abused kids? They hide injuries all the time. In fact, they’ll hide anything that has a chance of putting them in the spotlight of adults they think even might potentially abuse them, let alone the adults who actually do.

I think people forget that Harry was physically at times and definitely emotionally/verbally abused for the first 11 years of his life. He has a lot of responses that are conditioned into kids who grow up with abuse. He doesn’t want to be a bother because he’s psychologically predisposed to believe that doing so will make them care less about him and that’s the last thing he wants. It’s a lot of the reason Hermione and Ron both have such a decent relationship and friendship with him. They both in their own way go out of their way to show to Harry that they do care. And that they aren’t going anywhere (despite some growing pains during the GoF, and the bit in the Deathly Hallows where Ron storms off and gets caught by the snatchers. Despite the few times they don’t see eye to eye and argue they do show that they value each other.

But when you’re conditioned to think you aren’t valuable you go out of your way to protect the people who see value in you, even in misguided ways like saying nothing is wrong when everything is.