Sounds like your camera is really doing native widescreen by using extra pixels on the CCD to the left and right of the 4:3 image. For a test, does this camera have a still photo mode? If so, lock it down on a tripod, shoot a few seconds of 4:3 video, then a few seconds of 16:9 video and finally a still photo. Now compare the 3. If you overlay on separate layers in Photoshop and do some scaling so they match then you'll see what's going on. I suspect the camera only uses the center portion of the CCD in 4:3 mode.
I did something like this to figure out how the PDX-10 samples the CCD's in the different modes:
http://www.greenmist.com/dv/16x9/10.JPG