BackTesting Equity Graph

I am trying to understand what how the gray equity line on the equity graph works. I have programmed my bot to close all positions and not open any new ones when my balance reaches $1800. At this price I have an account equity of $2207. Why when the equity is above the balance does the balance decrease to $1500? When there are no open positions shouldn't the balance and equity be equal? 

In the equity equation (pic below) equity is given as 'Balance - Unrealized net loss + Unrealized net profit + bonus'. That would lead me to believe that when all positions are closed the balance should increase?

What is the Bonus?

Many thanks.