Quoting Alpha Male:
I believe it to be the temprature that the compressor kicks off, there's a temp probe on the AC evaporator, at a certain degree (cold) it kicks the compressor off and then back on, so if you're recirculating the air, obviously cooler air gets colder, as opposed to pulling outside air, hotter air not getting as cold, it will adjust how hard the compressor works, you could crank that thing all the way down but you might deal with ice build up.
Thats off my memory, it's been a long time since I've worked on the AC system in one of these.
This was my guess as well, but I'm not see this reaction. Is it only when in eco mode or for both eco and max modes?