This only assumes though that the game has been built such that players are only expected to win 50% of the time... which we all know is not the case.
Right, but now you're applying the analogy to the wrong aspect, because we're really talking about game balance as it applies between the various players.
In the analogy, the two teams are representative of each player's efforts and each character's ability to affect the RPG.
In an unbalanced game, like 3.XE* or Ars Magica, one player gets to play the Chicago Bulls, and a different player gets to play as Duke, and yet another player gets West Texas Junior College. Whenever the Bulls show up and decide to actually bring their A game, they win. They have to deliberately hold back in order for Duke or WTJC to be meaningful.
Or, in other words, in the beginning of a 3.X encounter, you know that, if the Cleric / Druid / Wizard decides to show up, that he's going to win the game - the Bulls trying really hard are going to beat Duke or WTJC, even when they're playing at their best. If, instead, the Bulls decide to just coast through the game, and do nonoptimal things like see how many half-court shots they can make, then Duke's got it in the bag. Etc.
Or, to maybe make the analogy a little cleaner, the standard adventuring party is a group of 12-year-olds playing basketball against a group of 10-year-olds (the adventure / combats). The 12-year-olds, by dint of two extra years of growth / practice / coordination, should beat the 10-year-olds just about every time (which is why pre-high school sports are generally segregated by age). The problem comes in when you realize that, on the 12-year-olds team, one of the kids is actually Larry Bird in disguise, and another is Michael Jordan, similarly hidden. In any game in which they decide to play their best, they'll not only beat the 10-year-olds handily, but their contribution to the team's score will completely outweigh that of the other kids.
* At mid-to-high-levels past the sweet spot.