I thought that was explained before...maybe you missed it. The changes in the number of shares available to borrow for shorting purposes don't necessarily result from the borrowing of shares. That number can go up and down for a number of reasons. It could go up because someone bought shares in a margin account....or down if someone sold shares held in a margin account. And yes, it COULD change because someone borrowed some of them to use for a short sale. There are probably other reasons that that number can change, but I'm not learned enough in the subject to know what they are.