Is this a Harry Potter mistake or can you explain?

i can understand how that confused you. i don't think it's a mistake just was worded wrong, i think it means most wizards in any other house usually don't go bad. but then what's your definition of bad? some might consider James was bad as a kid. then again (sorry if you have not read all 7 books) Snape was in Slytherin and he ended up being good.