I tend to agree that most Christians really don't know what they are. I''m sure most would just click a default 'nondenominational' tab which could mean any number of things.
Personally, while I do know exactly what I believe and why I believe it, it would be very hard to sum up my beliefs in a drop down menu. How many people do you know who understands what an 'Evangelical Continuationist' is? *lol*