What I observed is that my two Chase Visa cards (Sapphire Preferred and United Club) will bring slightly different rates even if two transactions are made almost simultaneously, one on each card.

And almost always, United Club card has slightly higher rate (about 0.1% to 0.2%), while Sapphire Preferred usually stick to the rates posted on Visa.com.

I will post some of the transactions here later when I have the time to dig through the statements.

@percysmith: Can I assume, based on your idea, that even though acquirers have control over the posting date, the rate used is still according to Visa/MC (on that day)?