The process used generates a statistical approximation and tells you how close it is to correct, in theory a perfect solution would beat this by that amount, in practice of course Poker is a game of chance, and so over any realistic game it wouldn't matter because the deviation from correctness they've computed is tiny. Could they make an even smaller deviation with more compute used? Sure, but why bother.
https://en.wikipedia.org/wiki/Cepheus_(poker_bot)
Cepheus is instructive also because some humans have played against this and believed they were outplaying it, which indicates there are real human poker players who misunderstand their own variance so much (and/or discount real variance from others so much) that they're completely unable to successfully rate their abilities.
If you lose 12Bb over 100 hands you are not, in fact, "winning except that it sometimes gets lucky". You're just losing, of course it sometimes gets lucky, that's how luck works, it's a game of chance.