[pliny]

>shouldn't it be roughly 50% gaining and 50% losing

1) They aren't trading against each other (they're trading against professional investors)

2) When you trade randomly you should expect to lose money (paying fees, paying spread, getting 'good' executions rarely and getting 'bad' executions often, etc.)

[3chelon]

50% losing and 50% gaining, without any spreads or commissions, would eventually destroy all your wealth. It's the classic "random walk" or Brownian motion study in finance, and it destroys your wealth because every time you lose 50% you have to gain 100% to get back to where you started.