My race bike doesn't do this because the clutch switch is gone (and it has aftermarket ECU programming).
But in the absence of being able to check for myself and thus having to take a wild guess ... Probably ECU programming. It's designed to do that. It raises the idle speed a little when in gear to make it easier to start off from a stop without stalling the engine. It's possible that the ECU takes other mysterious factors into account. My car (manual transmission - becoming a rarity) does this, too.
Idle speed regulation is not absolutely perfect; it's going to vary a little depending on how hot the engine is, how hot the oil is, the condition of the clutch (when in gear with clutch held), etc.
It's not adjustable, anyhow. There's nothing to adjust. Unless it is doing something crazy like idling at 5000 rpm, there's nothing "wrong". So ... put it out of your mind and forget about it, and let it do its thing.