You're seeing a range of rpm's because the converter mfg has no idea what the torque of your motor is at the flywheel. If it was, for instance, 400 ft/lbs, then it might stall the converter at 2500. But if the motor produced, for instance, 475 ft/lbs, then it might stall the same converter to 2800. More torque, more stall.

Don't hold me to these specific numbers, I'm just trying to help you understand converters and stall speed. Like Pat said, there's more to it than meets the eye.