I'd make suggestion that the reason they have been sub-chartering in during this transition period between Fokker/Embraer is because they've, back to back, had crews offline retraining on the Embraer, ground school, simulator, touch and go's, umpteen sectors line training waiting in the queue for the next Training Captain to become available etc.
I think that was pretty obvious all along Harry.
Here is the post from N707ZS which started the whole discussion:
Have they got enough aircraft to replace the retiring aircraft? Noticed an RJ-85 operating a KLM flight this morning.
Therefore the answer to N707ZS was that in fact, albeit on a temporary basis, no they don't have the required number of aircraft for a straight transition and therefore that is why they are sub chartering CityJet equipment.