Have you ever had a brain-fart pay off?

Close the door. Tryin' to air condition the neighborhood?
...Every kid has heard this from their parents. Now, I'm not too sure.

Well, as it happens...I have an outdoors shed I use as a growroom. It's about 8' wide, 12' long, and about 8' high with a crappy window a/c unit, (485 watts) and a 1' x 1' active (fan forced) vent near top of room, mostly insulated on top and sides. Fully exposed on all sides...no shade.

Normally, when outdoor temps hit around 107 (daily) I have been forced to turn off one of the two 400w hps lamps, raise and center the other, and crank the a/c on high...and still I'm only able to keep temps at around 95ish. I've learned how to grow in the heat, and am comfortable with my techniques, but a cooler flower room would be nice. :thumbsup:

Now for the brain-fart part:
On saturday morning, I was watering the plants (I turn on the a/c while I'm in there working) While in there, we had company come over for the day, and they left late in the afternoon. After they left, I remembered about my growroom door still being proped-open, and both lights were still on. :eek: (was 107 outside by then)
I ran out there thinking the worst, and panic was setting-in.
My only saving grace was that besides forgetting to close the door, I had forgot to turn off the a/c, too.
I stepped inside, and believe it or not...it was cool inside. Well, cool for an outdoor growshed in the middle of the desert. 88 degrees, 45% humidity. Almost acceptable, considering both lights were on.
-So-
I've been doing this for the past few days with the same results. Cooler growshed with the door propped wide open and both 400's lit. I just don't get it.
I know that with a swamp cooler, you have to crack open a window a little bit to vent the heat, so I figured opening the 1 sq.ft vent was adequate for the a/c unit. I guess not.

I'm soooo confused. To me, it just doesn't make sense.
Any viable opinions out there about how come this works?