I don't know about the radiator/fan layout on a CRV, but I think the difference in temp would be mostly due to a "safety" feature. For example, a 2nd fan that kicks in automatically when the AC is turned on, or if you got only one fan, it turns faster, etc. even if coolant temp doesn't require it. Just to prevent more overheating.
Just my take on this phenomena, I know *some* cars are designed this way. Pop open your hood with the engine running and ask someone inside the car to start the A/C while you check the fans. Maybe I'm wrong though.
But the coolant gaz return is still an interesting theory... I don't know if it'd be cool enough to make a difference in coolant temperature in the radiator. Maybe.