Due amount on your invoice can indeed differ from one billing cycle to another. It is calculated on the prorated basis and dependent on the number of employee licenses you have and how you've managed them during the billing cycle.
So, if you have activated or deactivated employee licenses during a billing cycle this will be reflected on the due amount on your invoice for the next billing cycle (by adding debit or credit, depending on your mentioned actions).
When calculating the prorated charges and the needed adjustment to the amount on the invoice for your next billing cycle, Insightful takes into account:
the number of employee licenses you have at the end of your current billing cycle,
the specific period of time during which you have used licenses that were subject to changes (activation/deactivation) in your current billing cycle.
You can learn more about how pro-rata works here.