Shorting bitUSD into existence adds value because it takes BTSX out of play that was otherwise liquid. Effectively reducing supply but not actually burning the share. This increases the price to obtain BTSX but keeps the supply multiplier the same and therefore marketcap rises. Shorts don't last forever though and it's conceivable that the average amount of BTSX shorted at any one time may stabilize as the market reaches a less volatile state.
As for their available supply and total supply those numbers will be the same because BTSX is DPOS and not mined. Therefore the entire supply comes into existence at once. This is different than Bitcoin where the theoretical max supply is known but only so many coins are mined so far.