I think it is due to the complexity of the gi game and the level of control you opponent will have on you due to the grips when wearing a gi. If you use a pass with the gi, quite often you will need to be technically better to do the same pass as your opponent will be able to use the gi and grips and general friction to stall your progess. Without the gi, the same pass can often happen more easily due to less control and the speed, especially when sweaty. This will often lead to a slightly less technical ability in the smae technique when comparing gi and no-gi. Please take note that this is an opinion developed from training with a gi myself first!