My experience is that it's just another internal dice roll based on how many steps you've taken, because each group (A, B, C) unlocks when you've hit a certain number of steps. My total for the day in Yellow Forest is 19,000 steps and I can't get anything past the second single question mark, which is something I've never seen happen before. Bad RNG maybe, but honestly I don't think there's any rhyme or reason to it.
edit: Also, Parasect is hands down the best capture bitch. Spore + False Swipe
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