Many kids can stunt way past their tumbling level. That's completely normal!
Everyone is different, so I can't say a blanket statement about tumbling. Without seeing a video of your tumbling, my first thought is that you weren't taught a correct backhandspring. If girls can do more advanced running skills, but can't connect skills after a bhs, it's usually not an issue of fear. Most of the issues I see are fear based. It was so hard for me to get my running hand tuck, I struggled with too much power and not enough control. Meanwhile I had my jumps to tuck and standing hand tuck because I felt more in control on standing skills. It took a bit for the fear of running to go away! Since it seems you weren't afraid, then it becomes a tumbling technique issue. My guess is your bhs didn't have the speed needed or the snap down needed to connect another skill to it. At a large gym you likely would not have gone past level two without progressing your skills.
As a coach, if a kid tries out with a round off tuck/layout/full I don't think "oh look at that tuck/layout/full" I think "are they not able to put the backhandspring in?"