Because even with no fees, the rate your card uses to turn USD into whatevers will vary from card to card. If you're not paying an up front transaction fee, you're almost certainly getting rinsed on the rate offered.
Even among cards that generally use the mid market rate, the time they set the rate and their source will often differ, resulting in the differences mentioned in the OP. So if you feel it's worth your time, and if you can get up to date rates, you can still save a bit by using the card with the best rate on a given day.