I think self esteem issues are part of this, but more often it just a way of getting attention
If a guy told me, hey Jen, you 31? I's totally sure you were 35, I'd just scold him for fun
...meaning the girl aint necessarily mad at you for real...maybe she was looking for fun? Lol, when she said 23, you shoulda said, with a big smile on your face...oh...I meant to say that, but I just wanted to make the age span go away