...I often wondered, though, whether it was better optics in the Rollei, or whether the mounting and changing mechanism of the Mamiya (which I always considered to be somewhat tinker toy) resulted in not quite accurate position of the lens to film, or slight differences in the focusing vs the shooting...
Fair enough.
This also brings up another good point, though...the importance of shooting a test roll of film, or two.
Ensuring that the lens focus actually matches what you see in the viewfinder or WLF focusing screen.

Taping the front page of a newspaper to a wall works well for me. Bracket some focusing shots,
much like bracketing for exposure.
These 'focus bracketing' shots will show whether your TLR 'taking lens,' is in tune with the 'viewing lens.'

Marc