This logic not only happen on the DASH 8
Every today turboprop engine have a free turbine.
You cannot set a max 1020RPM for every turboprop.
I work in the business aviation, this I know B300, B90, TBM.
Pilots drive their engine with Torque and RPM.
They don’t even look at the N2.
Example on B300, most of time of your flight you set a fix RPM around 1500tr/min (set with propeller pitch lever) and and then adjust the torque only. (With power lever)
Around 80 Newton.meter
It is quite strange to explain, but this is how they fly this aircraft. (