People in the setting tend to underestimate Bam (even if they know he's an Irregular) because he is so comparatively normal-looking and because, due to his lack of training before coming to the Tower, he started from zero.

But he's probably the most abnormal of them all once you scratch the surface

He shouldn't have been able to absorb those souls like that... And that's the point. The fact he can is an indication that there's much more going on with him than simple talent to copy skills he sees or a ridiculous natural gift.