It was used as Bob mentioned for the "corporation type bets" that are typical in 4 figure and above sets. For example: The player has none of it, and gets 25% cut if he wins. Backer A puts up 40%. Backer B puts up 30%. Backer C puts up 30%. That's all one corporation for one side. The other side does similar. All that money is pushed "in the middle". Side bets amongst side betters are separate. Side bets between one of the corporations and a side better are also separate.
With just 2 normal guys at the pool hall betting their own (or a single backer) it doesn't make any sense. With 12 total people involved, it is reasonable.
Still, I prefer the "per side" number to be the published one.