For me it's usually an attempt to not come across as staring. A lot of people grow up hearing "don't stare!" when it comes to people who look different because it's rude. So it's really hard for me to shake that. I don't look away because I'm weirded out by a person or because I think less of them. It's because I don't want my initial glance to be mistaken a stare. It's ironic that apparently that can come across as rude as well. Then again I'm socially awkward and struggle to make eye-contact with non-disabled folks just as much.