Transmission fluid change on 2002 GMC Envoy

January, 2014, dealer replaced transmission seal @ cost of $114, 60,000m
August 18, 2014, today, transmission fluid changed @ cost $200, 65,000m.
How was seal replaced in January w.o. replacing fluid? Was old, used fluid used?

Asked by for the 2002 GMC Envoy
Depending on what seal(you don't specify) , no fluid may have drained out ...was it an axle seal?