Thanks PeterHanes,
I figured it out. I just read the monomachine manual again. There it is made clear (page 56):
When in external MIDI clock synchronisation mode the Monomachine will stay in sync with an external MIDI clock if present. […] The tempo displayed in the tempo screen is only an average calculation. Even though it might appear to vary the Monomachine is tightly locked to each MIDI clock signal received.
So everything is fine since I have acutally not noticed any timing problems!
Happy! Happy!
Now I will go on exploring more sound territories.