it doesn't do that when you're asleep because the abrupt change will knock you out of Dream Land.
So, the machine moves slowly in order to not wake up you up.
This does mean that it makes for longer response times as discussed above for the reason
of using a pressure on the low end that is very near to your threshold of airway collapse.
This way you get a better nap time and when something adverse does happen the machine can run the pressure up and
lock your airways open, hopefully thwarting the Obstructive beastie.