The roll back when in gear with foot off brake enough so as to allow brakes to be released, is normal when the idle speed is low. The idle speed is variable on that car based on ambient temp. If temp is lower the idle will be below 700 rpm and the torque converter which links engine to input of transmission, is less effective the lower the eng speed. When torque converter effectiveness decreases the torque converter is not applying enough engine torque to input shaft of trans so as to push car forward with enough force to stop the backwards motion. At other times when outside temp is higher and idle speed is elevated to 850 for better A/C performance when idling, the rollback will be less pronounced. The key here is that you should hold the brakes applied while stopped regardless whether the trans has more rollback resistance or not.