It shows submission - and is an appropriate way for your GSD to greet someone above her in the pack - that she does not do it with you does not mean that she is not submitting to you. Probably more relaxed and comfortable with you since I'm guessing you are home with her during the day?
Gryffon Vom Wildhaus
BH, OB1, TR1, AD
Rottweiler/Hairy Dog mix?? 2004-2015