I'm 5'6 and a half, and I'm a girl. I always thought that it was at the higher end of average, maybe just a touch above, but people are always remarking about how tall they think I am. I don't mind it, I just think it's a little odd. I'm also pretty thin at 117 lbs, so maybe my skinniness makes me look taller than I am? Anybody else get this?

When I was really little, my doctor told my mom that he thought I would grow up to be about 5'8 or 5'9. Well, looks like he was wrong. :p And in elementary school I'd practically tower over people. Haha! People would say, 'you should get into modeling' or 'you should play basketball'. But once I got around middle school age, and into high school, I felt like I was more average, and maybe even shorter. People who used to be so much shorter than me, just sprung up, and then I was no longer one of the tall ones. :p
I think its probably a good height to be at. I think average height is probably 5'5ish. Either way, it doesn't matter much to me what height a person is. We all come in many different shapes and sizes. And a lot of times I've noticed how short people wanna be tall, and tall people wanna be short. Its whatever to me really. I guess as long as everything is in proportion with your body. ;)
Her legs make up like 80% of her body and we always tease her about it, so maybe you have really long legs?
As you say, body shape can add to the appearance of greater height, so being slender probably contributes :)