Well, looks like we have a fault in MAME.
DK uses uninitialized ram to seed with. Therefore, mame should simulate this by randomly twiddling the bits stored in it before booting the machine, and storing these random bits in the playback file, so playback still works. Wonder why they didn't think of that?
This also suggests that until this is fixed, the MAME category and the real board category should remain separate.
Pacman indeed has zero randomness, and thus a playback to real hardware would sync.