My theory would be that someone was pulling a prank on the driver…the car equivalent of putting a “kick me” sign on someone. 
I don’t doubt any of this, but as I say, in my experience, it doesn’t translate into bumper stickers. And if you’re a PhD-level scientist, it might be hard psychologically to advertise your support for someone as pro-creationist at Bush is, particularly if you’ve gone to the trouble to put a Darwin Fish on your car.
I’ve used these before. They’re static cling, so they come off easily. Haven’t had the guts to slap one on a stranger’s car yet, but my sister drove around for a few days before she noticed ;).
–FCOD
The Concourse of Hypocrisy has a great gallery of conflicting and hypocritical bumper stickers. It mostly concentrates on self-proclaimed environmentalists driving environmentally unsound vehicles, but it’s still worth a look.
OK, I guess I can’t argue with your opinion about bumper stickers. But I can assure you, there are plenty of people who see no contradiction, psychological disconnect, or any other problem with supporting a Republican candidate and believing in evolution, or any other scientific fact.
I saw one the other day that I’d only seen once before:
“I hate Barbie. That Bitch has everything”.
On a brand new Lincoln Navigator.
Oh, so he’s a televangelist!
I once received a window sticker from the Sierra Club, along with a letter saying “join us and put this in the window of your car to show your support for the environment!”
I don’t have a car.
Oh… i almost forgot about this one! I used to regularly seen an SUV with the sticker, “Less is More. Stay Pure, Stay Poor” parked in the driveway of a house in one of the most desireable neighborhoods in San Francisco (one of the most expensive cities in the US).
I feel the same way whenever I see one of those “Free Tibet” bumperstickers. I always want to pull up next to the driver and ask him how much of his car was made in China.
Oh, and speaking of contradictory bumperstickers, here’s an old thread from Fenris featuring a car that has a multitude of glurgy Christian sentiments, including the old, “Adam and Eve, not Adam and Steve” canard. And a rainbow pride sticker. 
Well silly, go out and buy one and put the sticker on it. Don’t you love the environment?
In the same vein, I once saw a car with an anti-abortion bumper sticker (and matching license plate) race through a school zone on a weekday morning. I guess the “save the children” mentality doesn’t apply when the kiddies are old enough to run.