It is not my site. So I don't know exactly why but from what they say in the disclaimer, it seems that the volume reported is collected from the volume reported by several brokers to a goverment agency and their total may not be the same as the actual total. Also, it said that the reported volume is net volume, whatever that means. Maybe it cancels out buy and sell the same day.

What it does is to give you a feel as to how much short selling is going one.