It's been cover in previous posts. The milage meter does take idle into the equation. If you look at your average milage meter while idling. You'll see it drop.

As for the real-time milage meter. Zero distance traveled divided by any amount of fuel consumed will always be zero. How can one calculate mpg or L/K without actually travel any distance?

Honda's milage meter is actually very accurate as long as one rememberers it always off by about 10%.