Grooming is actually a succession of movements.
from
http://www.ratbehavior.org/Glossary.htm#CCG
Cephalocaudal groom ("CCG"): Grooming sequence of face and body (common to all rodents). The rat starts by licking the paws, then rubs them over the head. This is followed by licking and rubbing the side of the body, the anogenital region, and the tail. The sequence may be anywhere from loosely organized to very stylized, performed in a similar or identical fashion each time. In rats, most sequences appear to be loosely organized. The grooming sequence may be interrupted at any point, and it seems that rats usually stop before grooming their tails.
Some rats have the cleanest tails...some have clean bases (they work towards the tip) and the rest is gross...these rats stop.
Has your girl always had a nice clean tail or is this new to her?