Just trying to keep the math simple. I could have used 90, 110, 95 and 105. The results would have been the same. The point was that getting an average return in future play moves the overall average result toward the ER of the game.
Even when a player loses his session bankroll they still had winning hands and their return might not be all that bad. For example, say their bankroll is $100 and they are fortunate enough to play 5 hours at 800 hands/hour on a quarter machine (4000 total hands or $5000 coin in). If they lose that $100 bankroll they will have lost a total of $5000 while winning $4900 for a return of 98%. Heck, they could have been playing a 97% game and did better than the ER but still lost their bankroll.