Because it relates to the CheckCPAP mode that you are using and has nothing to do with exhale relief you might use.
In the manual I have it's explained under "modes".
Essentially what that plus or minus 3 means is that the machine will take your AHI and make adjustments within that range but it doesn't auto adjust during the night...the change is for fixed pressure change.
Example...after so long the machine evaluates the AHI and decides if the fixed cpap pressure needs to be adjusted any.
It won't change during one night though.