V.W.
yep. I thought everyone did. I, too, would sometimes try to let them finish up the nap in the car if I could (when the weather was temperate enough, we are down south, so between April/May --October, leaving them in the car wasn't even an option). Usually, they would stay asleep for maybe 5 more minutes and then wake up. Or wake as soon as I carried one bag of groceries into the house. :(
I think part of it is timing, and unfortunately not something you can really control. There are parts of the sleep cycle that they can stay asleep even through you removing them from the car seat and then lying them down in their bed/crib, but ONLY if they are in the right part of their sleep cycle. Mine very rarely were in that part of it. The best you can do, is try to avoid having them in the car near their naptime so that there isn't a chance they'll fall asleep and not get the nap. Easier said than done sometimes, I know.
<hugs>