Omnis, I believe there to be no rational explanation for your froth troubles. I don't think the froth is supposed to do that. Like, was it still popping and fizzing as you drank it or was it done with that already? All I know is that Pepsi is terrible at getting carbonation to stay in their pop, while I can pick up a can of coke 8+ hours after I opened it, jiggle the can, and then I hear "sssshhhhhhhhhhh". It starts out small, then grows into a very drinkable symphony. Good as new. If I tried that with a Pepsi I'd end up spittng the leftover syrup all over the place. Coca-Cola OWNS!
And is RC of British origin? Maybe that's why it sucks so proficiently.