off guard states that you take a -2 to AC but grapple targets Fortitude...
does being off guard make you easier to grapple? do you take the -2 to you 'defenses' and thus suffer a penalty to the Fortitude save? or is it strictly just AC?
Off-Guard is just strictly to AC. Not to any of the saves.
Generally, the conditions do only what they specify. Not other things that feel like they should be related, but aren't mentioned.
For example, the Blinded condition doesn't penalize your Reflex saves. Immobilized doesn't penalize Reflex saves either nor does it prevent you from rolling them.