Short sales during the trading day are sales that MM's made (without shares to actually fill the trades).MM's have the authority to do that provided they "find" the shares to close out the day.
So if a certain percentage of sales were "short sales" on a given day,it means that MM's had to sell shares they didn't have to fill the demand.Then they had to "find" shares to make their "obligation" complete by the end of day.
that MM's had


The short sales reported at the end of day only reflects the "difficulty"that MM's had to fulfill trades without using "air shares".

Kelt