I can test mine and see, but that actually sounds right. Heat will rise, so by putting it out on the floor vents, it will warm your feet as well as the whole Jeep and rest if you as the warm air rises. If you put it out at the vent level, you won't get much around your legs and feet as far as warm air.
2013 JKU Sahara, Gecko Green, loaded up
Spidertrax spacers, Teraflex levling kit, 275/70/18 Duratracs, AEV front/rear bumpers and hood