I am aware of the fees, I started noticing this kind of manipulation in the rate showing on their screen only when the time bitcoin is falling...and its not for the first time I buy this amount of Bitcoin from them and they are not the only exchange I trade with ...
I posted this video for the ones who are blindly trusting any broker just because they are controlled by big players... You are totally right about the fees but not that much...I have another video that the gap is more severe :

And to me who is not a smart guy like you, and obviously not a perfect accountant to see these tricky "formulas" that you spotted ...I just see the amount that I put fiat and the Bitcoin I receive based on coinbase market rate...