AIUI, what you're missing is that it's an iterative process. Energy comes together and forms a hair more matter than antimatter, and the rest returns to energy.

AIUI, what you're missing is that it's an iterative process. Energy comes together and forms a hair more matter than antimatter, and the rest returns to energy. But this wasn't an "exactly one chance" situation; the energy gets a fresh chance to come together and a bit more of it is matter than antimatter, with the rest going back to energy... and so on and so on. If it was perfectly balanced, no amount of iteration would produce matter, but even slight imbalances end up tipping the scale greatly over a lot of iterations. (And of course it was all just a big mess of things, not a clean series of events like my words imply. But that's the idea.)