In many cases (and I'm not saying this applies to you), this is due to the person not being very good with "other casting methods". So when they switch to the fly rod, they do about the same or maybe even better.
I'll say it again, though -- I'm not saying this is true in your case.
This could be because the spin/baitcasters are just not very good at it.
Every method has its day; however, it's not the tackle that determines success, it's the fisherman. A good spin fisherman could outfish a poor fly fisherman and vice versa. Knowledge and experience trumps tackle.