Could be a plugin. I have one installed called "NoSquint" that can resize pages on a site-by-site basis. I have Bikeforums set to 75%, since the interface is, by default, so completely huge.
I'm sure it could be set up to behave as you describe.
Other than that, if you're comfortable poking around in about:config, you could search for strings that contain "zoom" and see if anything looks odd or non-default.
Found it. Someone had run the zoom-out command on one of the WWW pages. Oddly enough, it didn't carry over to the page variants without that prefix. Resetting the zoom on the WWW-prefixed site did the trick.