The older models only supported 4 players. So that is where the 2 bits came from. In the newer models it seems they wanted to be able to support more players, so they just stuck an extra bit onto the end of their previous format. I am guessing they did this for backwards compatibility with the older models (which would just ignore the additional bit).
So, theoretically at least, the newer models could actually support up to 8 players.
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