Just as you have rightfully stated. They will be losing but may be, sometimes they will lose very small, some times they will not lose at all. Yes, may be 80% of their trade is what they may make profit from and lose 20% .As Human those professional traders must be lose their money at least once, and if they create lose i believe that mostly or at least 80% of their trading must be profitable for them, so in this case they are still make profit in forex even though they lose their money sometimes.