Posted by KBPsen:
Because the CPC calculates a target cabin pressure based on what is set on the cabin pressure control panel. This will then result in the expected differential pressure if you are at the same flight level as selected on the control panel. Go above the selected FL and the differential pressure will increase. Basic stuff really.
Agreed. The last paragraph in the FCOM where it talks about the cruise phase of flight reads:
Deviations from flight altitude can cause the pressure differential to vary as the
controller modulates the Outflow Valve to maintain a constant cabin altitude.
So, I think the computer looks at your FLT ALT and then says, "Ok this is the target PSID I will give them when they are there." (eg. 8.35PSID) Once within the 0.25PSID of FLT ALT, it goes into a "alt hold" type of mode where it now varies the PSID to keep a constant cabin alt.
If you go above FLT ALT, then the computer can't keep the cabin alt constant without increasing the PSID.