There are 3 types of admins as explained in this article. You are billed according to the highest tier that an admin was in in a specific payment cycle.
I think an example explains it best:
Let's say your billing cycle is on the first of every month. Let's say you have a full time admin called Joe in your system. As part of some changes in your organization on the 10th of the month you make Joe a part time admin. At that point, you will see that Joe is pending part time until they log in again in which case the system will log them out and log them in again with their new abilities. Since Joe was a full time admin part of the month you will be charged according to the full time tier for that admin regardless of the fact that they are now part time. We thought it makes sense to allow you to change an admin level at any given moment and not only at the beginning of a cycle. In the next payment cycle you will be charged for Joe according to part time admin as long as you do not upgrade them.
Note: If you upgrade Joe in the second month back to full time, even if it is on the 29th of the month you will be charged for him according to the full time tier. Generally, it more sensible to upgrade admins as close as possible to the beginning of the payment cycle and to downgrade them as close as possible to the end of the payment cycle so they will enjoy the extended abilities for as long as possible.