AC turns on without being asked

In my 2004 6.0, the air dial includes (among others) choices for AC max, AC, upper vents, and upper+lower vents. Cold air comes out nicely when I select AC max, or AC -- that's working just fine. And When I select upper vents, the air is not chilled -- which is also fine. But when I select upper+lower vents, the air is chilled: the AC compressor must be running to do this, even though I didn't select one of the two AC positions.
Could someone with a similar rig test this, and let me know if it's an intentional design feature? I can't think of a reason for it -- it's not a setting you'd use for defrost, and it runs the compressor unnecessarily (though I can add heat to combat it).
I've only noticed this in the past month or two, on this 10 month old F250. Maybe it's been doing it all along.
Thanks.