Some buses are more fuel efficient than average; so are some cars.

I was not promoting specific numbers; the point is there's a tradeoff and a tipping point where more trips make things worse, whatever the numbers are.

Averages have a way of being a lot lower than peak numbers, too.

Yeah, but the averages don't matter as much as the max in this case. The total carrying capacity between 8-10 and 4-6 is what's important, as traditionally, this is when most of the people moving occurred. It doesn't really matter if the buses are less efficient than cars the rest of the time, as long as they take enough of the overall journeys at peak times that the whole city traffic system moves quicker.