This happens because Mamesaver verifies all ROM's on rebuilding the list. If a ROM does not verify properly (ie if MAME does not have a "WORKING" label for that ROM), then Mamesaver ignores it. This is done because MAME asks you to type OK whenever you start a non-working the game.
As far as I know there is no way to turn off the OK question when starting a non-working game. If you know a way around this then please let me know
EDIT: If you check at MAWS (
http://www.mameworld.net/maws/romset/ssriders) you'll see that the driver for Sunset Riders is "imperfect" which means MAME would ask you to type OK whenever starting the game.