Idle CO is just that does not affect the main CO much at all, but as it adds to it it will have a very small effect, and very small it is.
If you imagine the air demand at idle and WOT you will see the point.
It will have a noticeable effect at low rpm low demand, much more responsive at 2/2.5% as it is running much closer to idle type air flow conditions, so percentage wise has more effect the lower the air flow.
tabetha