2
The code that produces those two values is in modules/system/admin/findusers/main.php:
$acttotal = $member_handler->getUserCount(new Criteria('level', 0, '>'));
$inacttotal = $member_handler->getUserCount(new Criteria('level', 0));
The inactive user count is the number of users with xoops_users.level = 0, which (I think) are users whose registration hasn't been been approved/verified (whatever the term is).
If the admin page doesn't help you identify those users, you can use the MySQL query:
SELECT * FROM xoops_users WHERE level = 0
