Does the Autopilot switch to ALT ACQ mode at a fixed altitude difference from target altitude?
The simple answer is that autopilots are designed to fly the aircraft in the same way that a human pilot would.....only more accurately!
If you were climbing/ descending at 3500fpm you’d anticipate the level off earlier than if climbing/descending at 1000fpm or you’d risk exceeding g limits and it wouldn’t be a very smooth transition.
Autopilots, including the 737NG, take into account the ROC/ROD to ensure a smooth transition to level flight so yes ALT ACQ engagement will vary.