NPT means tapered threads....the threads get narrower as you go farther in. NPSM (straight pipe threads) have no taper, same width the whole way.
This means that NPSM will only thread a bit into NPT threads, until the NPT threads taper too small to allow the NPSM to go further. Most of us just use a lot of teflon pipe tape and it doesn't leak.
There seems to be some variation in how straight NPSM threads are, and how tapered NPT threads are, so some elements fit into some NPT bushings/caps better than others. It's a crapshoot.