It's going to depend on what game you're playing and how you're playing it. Consider a blackjack card counter to a blackjack hole carder. The counter will have much higher variance, therefore a lesser winning percentage.
As to your second paragraph. You're not understanding what "playing with and advantage" means. It does not mean you'll win more hands than you'll lose. It means that the payoffs exceed the risk involved. If on a fair 50/50 coin flip, you bet $1 and only lose the $1 when wrong but win $1.01 when right, that's paying with an advantage even though you'll only win half of your bets. Another example is a flush draw where it pays 6 credits (Ala 9/6 JoB, DDB etc.). I'd take that hand every time even if high pairs are losers as the payoff exceeds the risk. In this case, I'm still losing the majority of hands but the few winners more than make up for the losers. Does this help?