See, apparently, he didn't believe I was American. I've had a Canadian border guard think I was Canadian on the way up once, before asking citizenship, just on the grounds that I knew how to pronounce 'Vancouver' correctly, but that was pretty minor. This time, it was a little surreal.
Now, keep in mind he had my driver's license (issued in Washington state), my birth certificate (which said I had been born in Seattle), could see my car's license plates (Washington plates) and looked up the registration during all this (which is a Seattle area registration). Yet for some reason which eludes me, he was convinced I was a Canadian trying to sneak into the US, and so wished to deport me back 'home' to Canada. My theory is that it must've been because he was rather rude to me, yet I tried to be unfailingly polite to him; evidently, only Canadians can be polite when snarked at or something.
In the end, he asked me questions about Seattle landmarks and history to see if I was actually familiar with the city, and finally -- faced with myriad bits of evidence that despite my increasing disgust with my country's border patrol I was, in fact, American -- he let me go on through and head home.