There are two things that exist outside the regular trading: First, exchanges skim a fee off the top of every transaction. That removes some money from the ana

There are two things that exist outside the regular trading: First, exchanges skim a fee off the top of every transaction. That removes some money from the analysis, causing a negative sum. Second, miners are a special case: They do not deposit any money, they only withdraw it using freshly minted coins. Thus they too skim off some of the value in the market, and also cause the sum of the regular trades to be negative.