Automatic transmissions with vacuum modulators have their shifting significantly affected by how the engine is running. The more power the engine developes the less need for throttle to achieve speed. Less throttle translates into more vacuum.
Thus if it takes about 15 horsepower to maintain speed at fifty (this is the average value we use for sustained speed on a dyno) and ones accelerator is at 15 % then one will have more vacuum than if it takes 20% throttle to make 15 horsepower.
Bosch Master, ASE Master, L1
33 years MB technician