Weird. As GRJ says, they should be the same. Unless your SD70ACE was somehow entered into your CAB-2 as a "switcher" (in which case, speed steps are halved.) Do you have a third engine to compare, to see which of the two is running "right?"
What happens if you place them both on the track (widely separated, with the GP-7 in front.) Use the "standard railroad speeds" and set them both to medium speed, etc. Do they even out after a while?