Your invoice amount is calculated based on several factors. A distinction can be made between the stock synchronisation and order processing modules. Both modules are calculated on the basis of the number of (sales) links and the number of orders you have run in the past month.
So it may well be that you receive more orders at the end of the year during the holidays and your invoice amount will be higher in January as a result. Because consumers often order less in January, your invoice amount will probably be lower again in February.
You can see the quantities you have been invoiced for on your invoice. In the invoice description below, you can see that this invoice is based on 81 orders, 1 sales link and 0 employee accounts.