It seems that if you create a active directory group that already exists, it will use all the same settings as it had before, but when you save it, it will get assigned a new order#, and the old number will disappear. Once that number is gone, you can't move any group to a number lower than the missing number, and you can't move any groups above that number either.
Example:
Say I have groups 1-9. I create another group that is the same AD name as group 5. Once that is done, it will create a group 10 with the same settings that 5 had, and the number 5 will disappear from the order. I can rearrange 1-4 and 6-10 how ever i want, but I can't move anyone from 6-10 to a higher order than 6, and 1-4 can't be moved any lower than 4 either.