I imagine the majority of people don't fill out this section actually (don't quote me on this though cause I could be wrong). Or if they do, a large portion of them really don't have an entirely valid reason for doing so. They simply do it cause it's an option which they think will improve their chances of being admitted.
I know for myself anyways, none of the four categories really applied to me, and I wasn't about to make up some poor excuse for why I was "disadvantaged" during my undergrad (although I understand that is not the only reason why someone may fill out Part B).
Don't get me wrong though, that's not to say there aren't many cases where people don't have a valid reason! I have heard some pretty crazy situations where the individual has every reason in the world to fill it out.
**My apologies, forgot this was the accepted thread, please delete