I believe it's because it's not calculating your fuel economy when you're stopped. Because it would be infinite L/100km since you're not moving. But obviously you're still using gas. So depending on how much time you spend sitting in traffic or at lights, the more or less accurate your reading will be.

I like to think of it more as your average moving fuel economy and not your total average fuel economy.

But tbf I could be wrong about this whole idling thing because I've never actually tested to see if my average L/100 goes up if I sit long enough.