Active Field Tools Users are counted by users that have access to the Field Tools portion of the portal as shown on the User Access page.
If the Active Field Users count is higher than you expect, unchecking all of the Field checkboxes for employees that should not have access will lower this count.
Active Field Users are also counted from anyone on the User Access page that have a check box checked under the PO drop down selection.
Unchecking these boxes should lower your Field Tools user counts as well.
Active Field Tools users also count any Admins on the Admin Roles page of the portal that are Training Admins.