Keying on what Dawei said, setting CPAP pressures based on one night's observation and titration in a strange environment, in a position you may not naturally sleep in, with all kinds of boondongles stuck to you, is kind of, well, kind of haphazard. I would argue that to get a good read, you would need several nights of titration in your own bed, without a ton of electronics hooked up to you. That magi pressure number can change from day to day, with a hundred variables feeding it.
Big meal before bed? A little nasal stuffiness from seasonal allergies? Big cold front coming through making that arthritic shoulder ache so that sleeping on it is uncomfortable so you have to sleep on your stomach instead of your side? Fluctuation in weight?
I did the one night titration with my first machine and it worked pretty well for me, but over time as I gained weight, I found that Ihad to bump the pressure up from 9 to 11.
It's really good to have the option of both apap and cpap on a machine. It gives you more options for adjustment during the year where conditions may change. I went 5 months with no problems, then bam, furnace goes on and I've struggled most of the winter. Doing some suggested experimentation, I switched to straight cpap for 2 weeks and that gave me a lot of relief. The problem with it though, is at the higher pressure, it takes ages for me to fall asleep, so what I gain from better therapy for the moment, I lost due to fewer hours of actual sleep.
I have now switched back to apap but with a tighter range (9-11 instead of 8-12). Once the heat is off, I will just go back to my previous settings of 8-11.2). You may not experience this sort of problem, but I'll bet that there are a large percentage of people, that if they knew how to do their settings, would be fiddling with them to improve their comfort and therapy.
FYI- my problem is low oxygen from a slow heart rate. I only have apnea or RERA events, untreated, during near REM and REM - so 1.5 - 2 hours a night.