So even though general Grievous loses a lot he apparently wins a lot of battles off screen. He's not losing all the time, just when he's up against Anakin or Obi-Wan. This was apparently the writer's explanation for why Grievous was never fired.
Also it's pretty cool that C3PO covered for Anakin and Padme, "Jedi robes, I don't know who those belong too, could be anyones." Nice job C3PO.