I am no expert, but I would definitely say that it is most likely just "normal." I do not think you should doubt what she has told you purely based on her anatomy. All women are different--shapes, sizes, smells, tastes--it's just the way it works. I am a female, and even after having sex plenty of times, I still fail to "loosen up." (And while that sounds great in theory, it does have its cons) And that certainly doesn't mean that I'm a virgin, I'm just small. If I were you I would work on developing trust and intimacy in this relationship, and use what you find there to determine if this girl is truthful and honest, not base it on some physical finding. I hope this helps!
P.S. It's not a good idea to tell a girl that she is "looser" than you had imagined. Just a little advice :).