Chances are it actually has less than 200L - the fuel takes a little while to settle in the tanks, so it's not uncommon for us to be able to get 5-10L more into a tank that at first glance appears full
What is the mechanism behind that?
I have never seen this in the TB20.
Sure if you fill it to the brim, and the fuel is say +10C out of the ground, and the wing warms to +30C, then the stuff will expand in volume by 2%, and if the overflow vents are located right above the max fill level, you will lose 2%, which is 4 litres, which in my tanks is equivalent to about 20mm on the fuel level (I have measured it).
But I have never heard of fuel "settling".
If the DA42 has both, how does this work when only
one readout is evident (IIRC; I have only ever flown as a passenger).