The 32 degree temp is what has to be avoided. Ideally the evaporator would be held just above the barametrically calculated freezing point of water (32 deg F at sealevel). When your evaporator is at 33 deg the air blowing over it is never made as cold as the actual metal of the evaporator. The sensor is measuring the temp of the air right as it comes through the evap. I would guess that in your case where the air is 39, that the evap is very close to 32 if not cooler.
This relationship (the increase of temp related to distance from the evaporator)is further realized if one watches the calculated temp on the PBU and also monitors it with a digital thermometer at the center vent. If you are cycling at 39 then you will see 42-47 degrees at the center vent. This changes as the car cools off. The first cycle from at dead start with a hot car can sometimes look like 50-55 degrees. That is because that 39 degree air is heated by the 130deg temperator of the dash and ductwork.
It also must be noted that in hot southern climates the A/C may never cycle around town, ESPECIALLY if run in the high blow position.
Bosch Master, ASE Master, L1
33 years MB technician