That is all pretty common stuff.
Commanders (well, other than the 900/1000) tend to have fairly leaky cabins, so the airflow in matters a lot. The more heat you ask for, the more air flow you will get, which can result in a decrease in cabin altitude. The opposite is true for asking for more cold air-- you will get less air flow.
It may be a sign that your cabin is starting to get a bit leakier than normal, not a big deal-- often it's the rudder pedal boots which are at fault, although any good Twin Commander shop or service center should have no issues trouble shooting and improving it. But to some extent, it is just how it is, even with the tightest cabin.