Cookie,

In simple terms, they are assuming that a buy was a buy because the offer was at or above the ask price.

They would assume a sell at a transaction below the ask price.

I did not see the total volume traded but assuming the 19,036 shown as a buy and the 68,800 shown as a sell, it would reason that a total of 87,836 shares traded hands.

Note : in a more heavily traded company the buy / sell theory works better.