I've noticed the same thing. It's like the software saves this thumbnail image at some random particular point in time, once, and that's it. There are times when this image appears all funky, like when you might get a bunch of errors or feature failures due to the part visually looking messed up. Then the thumbnail type image stays like this for the life of the file, even if the file is copied and pasted as another revision or saved as a copy for another revision.
SolidWorks orients the file to Isometric view and zooms to it's extents to generate the thumbnail on a white background. It does this during a save, after rebuilding the file.
The mechanism isn't robust and appears to work best on files that are opened in their own window in SolidWorks at the time of save.
My guess is that if you Zoom to Fit the model and orient it to isometric view, it will look like the thumbnail you're getting.
SolidWorks changed the behavior in 2007. Before that, it would just do a screengrab of the model with it's background, downsampled to 256 colors.