The mechanism has a spring loaded finger that pushes the date indicator past another spring loaded jumper. The fact that you are relying on two springs to act the same way every time, along with slight variations in the teeth of the date indicator wheel, means that some variation is possible. However I would not expect it to be a lot - couple of minutes maybe.
If the variation bothers you, try setting the hand top a safe time (6:30 is always good) and pull the crown out to the rapid date advance position (position 2). Then run the date wheel around the full 31 days a couple of times - if the lubrication on the teeth was not evenly distributed this will do that, and it may improve the consistency of the date change.
Cheers, Al