Your Account Balance does fluctuate based on the dates and amounts of orders for which you've received credit. This information can be found when logged into your account, by going to your Advisor HQ > My Reports:
Then, clicking in the Earnings link found at the top.
Also, there are also documents containing helpful examples of why your account balance may change found under the "Advisor Basics" links:
By clicking on the little graph icon next to the words "My Account Balance" you can view the dates and amounts that your balance will change. Hovering your cursor over the 1st line on the chart will show you the soonest date your Account Balance will fall.
For example, if you (or a customer) had placed an order last July 4th, from which your Account Balance grew $50, that amount would be "falling off" of your Account Balance this July 4th unless an order of equal or greater value is placed on (or before) the 4th. This information can be used to anticipate slow sales and help you to plan orders.
View the video below for a full overview of Rolling Account Balance as well as how to find what values are "falling off" in Advisor HQ > My Reports.