I think they just use the uptick/downtick rule. If the last time the price changed it changed up, it's a buy. If it moved down, it's a sell. Pretty simple really.
edit: this is on the trade page: "Trade definitions are based on the mid-price and are indicative only"
This could mean that they are labeled by whether they are closer to the bid or the ask. Bottom line, it's really not significant.