That is exactly my gripe about that tool. It is useless. When you see it showing as a buy or a sell, the software is "assuming" what the trade was. Here are the actual help descriptions - notice that they use the word "considered":
The 'Buy', 'Sell' and '?' columns - Where the 'Price' is closest to the 'Offer' price, the trade is considered a 'Buy'. Where it is closest to the 'Bid' price, the trade is considered to be a 'Sell'. Where it is dead centre between 'Bid' and 'Offer', it appears in the '?' column; this indicates that the trade cannot be identified as either a 'Buy' or a 'Sell'.