Why MOV on Import?

My typical workflow is to let FCP keep everything in libraries, so I copy on import. I've always assumed that it JUST COPIED my files, but I've noticed that they are rewrapped from .mp4 to .mov. The file sizes are about 10% smaller as well. I don't do anything on import, no optimization etc; I just like to copy to the FCP library directly from the card or drive.

This change from mp4 to mov is an issue because if there are any clips I'd like to stabilize in Catalyst Browse, that functionality is lost. I recently discovered this much to my frustration. 

I use FCP because of the easy workflow, and the "culling" I do before I ever get to a timeline, by marking favorites and rejects in the browser. It would be at this time that I'd note clips for stabilization CB. If I have to change my work flow to look at clips prior to import, I actually lose a lot of the reason I edit in FCP.

Is there a way to maintain my workflow and keep the files untouched on import. Furthermore, I'd love an explanation on what FCP is actually doing during import.