Many brokers truncate trading histories, particularly on demo accounts. Older trades may be replaced by an opening balance which reflects the total P/L before the truncation, or the orders may simply be removed from the history leaving a statement where the closed trades no longer reconcile with the balance.
There is a special consideration with MT4: the full history may be available, but it may not be downloaded by default in the MT4 platform, and therefore may not be published to the website. If you are using our publisher EA for MT4, and the website is not showing your full account history, try the following: go to the the "Account History" tab in MT4, right-click over the list, and choose "All history".