I checked my Left Difficulty - it was positioned halfway between A and B and I notice it seems loose. We had a bunch of toddlers over on the weekend and my girlfriend did mention she had to put my Atari away...
Anyway, you must be right - that 1st game must have been played on A. My theory is when I hit the Select switch to change game options, the slight jar+gravity must have been just enough for the finicky switch to toggle from A to B, thus taking the game out of test mode from that point on. Thanks.