I looked at the site that you linked and I see why you are asking. That's confusing to me. Here is the site I'm using, which is also used in papafox's thread:
tsla | Volumebot
There is probably an obvious answer for the difference between the sites, but I'm not sure what it is. I do see the trading volume is different. The site you are using shows double the volume that is shown for trading volume on SharpCharts and Yahoo. On the Volumebot site, the volume is half what is shown on SharpCharts. Perhaps it is the difference of whether a single buy/sell transaction is counted as 1 or 2 trades? On the Volumebot site, I think only sales transactions are counted. That must be what is going on. If you use just sales, then percent of sales that were short is 4x what it is on the site you are using. Probably what is most important is the relative change in the percentage of short trading over time.