Why are there some extra letters on lego Car License Plates?
The letters typically represent the initials of the set designer. See more discussion on this topic in this EuroBricks forum thread.

For example, the initials "MB" stand for Marcos Bessa, where "MS" is for Mark Stafford.

Some other sets also carry set designer initials, sometimes subtly incorporated in the build (I believe Jamie Berard, the famous Modular Houses designer, puts his initials in some of the builds), other times on the lettering on the sticker sheet. It is definitely not standard practice.