People spend so much time at school, not actively trying to, but learning what's socially accepted. If a kid does something wrong, the rest will make sure to let them know it, most of the time in a negative or embarrassing way which - despite not neccesarily being the right way to teach things - definitely makes people remember. That knowledge is taken for granted and others are expected to have it.

Those that spend a lot of time at home with their families, people that won't judge them harshly and will be more accepting of whatever 'weird' things they do, don't always learn what's socially regarded as strange because no one has told them, or if they have, they've done so in a polite manner. This is a nicer way of going about teaching, but doesn't always instil knowledge as well as being brutally honest or ridiculing someone. Embarrassment really works to make people remember what's right and wrong, albeit unfortunate for the person.

So many homeschooled people are regarded as odd, because they simply haven't learnt to socialise or deal with how people act.