The answer might be quite simple:
99.9% of the traffic going eastbound through the sector you are talking to goes to REDFA or SOMVA. Very rarely does anything turn right at CLN and go towards COA. So unless the controller is on the ball, he/she might not notice your planned route and automatically send you to REFDA. It is only when you query the clearance that the penny drops.
By the way, the airways are shown on the controller's strips, but there are so many different combinations that it is not always obvious which way the aircraft will go in practice.
We also have exit codes on the radar which show us where to point the aircraft. Via REDFA is it AM (for Amsterdam control), but I can't remember which it is via COA (AS or EB depending on destination?).