Putting on my magician's hat for a moment, that sounds like a magic trick to me.
Given your description, the simplest answer consistent with the laws of physics is that another player switched the dice when you weren't looking. Perhaps you stopped the game briefly to take a restroom break or answer the phone or deal with some other interruption. Your memory tends to edit breaks like that out of the narrative flow, especially if they don't seem relevant to the story. Somehow, the other player had the opportunity to switch the dice. Dice can be gimmicked in a variety of ways - they could be weighted, shaved, or simply printed with the wrong dot pattern - using a die cup wouldn't interfere with any of these. You'd played the same opponent before so he knew which type of dice you used; he could have brought fake dice of that type with him, swapped them in during the now-forgotten distraction, and swapped them back later. It's even possible the two friends were working together to play this joke on you, with one providing the distraction while the other made the switch.
At the moment he looked at you and said "not this time", the switch had already been made.
re: Magician's Trick
My friends had the opportunity to trick me since we regularly played Risk (and I would have been highly amused if they had done so). Since the dice were mine and were distinctive they would have had to get trick dice that matched my own. Then they would have had to wait for the right game opportunity, e.g., my 26 armies against my opponent's last remaining army on his last territory. Knowing my friends very well, it doesn't seem likely to me that they would go to all that trouble and then never laugh about how they fooled me.
My friends ...
What do you believe that most people on this site don't?
I'm especially looking for things that you wouldn't even mention if someone wasn't explicitly asking for them. Stuff you're not even comfortable writing under your own name. Making a one-shot account here is very easy, go ahead and do that if you don't want to tarnish your image.
I think a big problem with a "community" dedicated to being less wrong is that it will make people more concerned about APPEARING less wrong. The biggest part of my intellectual journey so far has been the acquisition of new and startling knowledge, and that knowledge doesn't seem likely to turn up here in the conditions that currently exist.
So please, tell me the crazy things you're otherwise afraid to say. I want to know them, because they might be true.