If the synch doesn't match, and the leaf shutter is only part way open when the flash goes off, the film is evenly exposed, but the shutter will perform like a second aperture and potentially reduce the brightness of the light that hits the film.
Sort of the same effect as changing from f/5.6 to f/11.
And the Mamiya 645 line cameras have focal plane shutters. There are a couple of specialized leaf shutter lenses you can use with them, but most of the systems lenses don't have a shutter.
Whoops... *embarrassed* I knew that too haha why do I even bother talking...