If you have multiple deposits, or deposits and withdrawals, then your percentage returns (total, per month etc) are not simply your trading profit divided by the total of deposits.
For example: you deposit $5000 and make a loss of $1000 (20%). You then deposit another $6000 (giving a balance of $10,000) and make a profit of $1500, i.e. 15%. You have made a cash profit of $500 overall, but your percentage return is negative: you lost 20% in the first period and only gained 15% in the second. The site will calculate this as a net loss of 8% (0.80 x 1.15 = 0.92).
(You should also bear in mind that profit and loss percentages are not symmetric. For example, if you lose 20% - e.g. $200 of $1000 - then you need a profit of 25% to return to break-even - a profit of $200 on the remaining $800.)
