Margin balance vs. account balance

I have a demo (mini) account with forex.com. I have been trading for a few days with an initial demo deposit of $500. Now I see the following account summary:

Acct Balance $483.00
Realized P/L $53.51
Unrealized P/L $0.00
Margin Balance $536.51

I have no open positions. Why is there ad difference between my account balance and my Margin balance?