I recall doing that as well. I'm pretty sure it was set up by Pepsi, to convince people that their product was better than Coke (ie, when lifelong Coke drinkers picked Pepsi). Like you, I found the tastes sufficiently distinctive that I easily knew which was which. So when I took the challenge and they asked which I preferred, I'd point to the Coke and say "Coke" - and I said it in such a way that they heard the capital "C" in the Coke, so they wouldn't be confused with the generic coke and think I meant Pepsi.