[jaywalker]

I think it may be due to the alphabetical order of the formations. For example, if unit B3 is reassigned to formation F then it takes the place of the orignial F0 unit, simply because B (its original parent formation) comes before F in the alphabet. Conversely, if unit F2 is reassigned to formation B, then it becomes the last unit in that formation, again, simply because F comes after B alphabetically.