The credit card cartels will get you any way they can. They do foreign exchange at bank rates, i.e. the selling rate is several points higher than the buying rate. This is why you see the discrepancy. Also, depending on the timing of the purchase and refunds, currency fluctuations come into play - might be more, might be less. Finally, there is a hidden commission of ~2.5% on foreign currency transactions processed on the major credit cards.

Frankly, the whole thing is far more of a scam than airline FF programs, but the cartels are clever enough to hide it better than the airlines do.