It's not erroneously. It is as simple as it gets. If you look at that chart you see that each trade at the ask price is listed as a buy. Each trade at the bid price is listed as a sell. The indicator simply shows the total buy (positive) vs sell (negative) trades.
How you interpret this fact is up to you.
If I buy an apple listed at the ask price in the shop, It's a positive trade for the shop. If I bid lower for the apple than the ask price & the shop owner agrees, it's a negative trade for the shop (in this case because he made less profit).
News 
Market Data 
Discover 
