People with better cameras might be better photographers (and/or more affluent, which might also show up as a plus somehow in some pictures in the form of clothes, dental work, hairdo, self-confidence).
Young, trendy, outgoing people concerned with looks might be over-represented in the iPhone category, while Andriod might attract slightly more technical people. If I look around at who is the first to show off their iPhone around here, it's usually those who want to appear trendy and good-looking, i.e. wanting to be noticed and approved socially. This is of course a gross generalisation, and I'm sure these statistics mean little to nothing.
I'm not a fan of Apple, but they are brilliant in selling quite expensive high-tech products to people who have no interest at all in technology and who would normally not even think about spending that amount of money on computers or gadgets.