As I understand my S9, it does fluctuate to my need for oxygen. It adjusts the pressure to the apneas I have. The nurse made an increase to my pressure limit to compensate better.
If you are being awakened by this feeling of being over-inflated (I was getting chipmunk cheeks with my incorrect mask fit) please call your clinician, take the machine in again, and be rechecked. If you continue to struggle with this, you might just stop using the CPAP entirely which is not a good option.
hang in there, L