Looking over the Transaction Details from a recent backtest, I noticed that my algorithm's daily cumulative buy-sell margins are small (never exceeding 20 cents), and usually positive. Yet, for some reason, my Daily Returns range from $0 to -$18 (note the minus sign), and consequently, my portfolio value drops by large amounts. I've ensured that there are an equal number of purchases and sales each day. Am I overlooking something obvious, or is this a known bug?