Are you sure you set the reciever pack to 4 cell on the Accu-Cyle?
BTW I have several GWS packs and I have yet to get one to work at rated capacity, they are all at least 20% less than they are advertised to be (I wont buy another GWS pack)
1 more edit: I just thought, and especially with NiMH batteries, some batteries do not perform well under high current draw and you may be getting a bad reading by discharging at 500 ma. Try a lower setting and see what you come up with?