Sorry...one more biological female weighing in:
I've dealt with that darn tummy pooch since puberty. I've always been thin, BMI around 18, 19. I have wicked strong abs, but, as you describe, you can just barely see the muscle definition on them because of that little layer of belly fat. I mean there are some olympians and marathoners etc. who don't have it, but perhaps they have higher T levels than your average gal...or models who are ridiculously thin.
I guess I'm inclined to say it's a girl thing 🙂