I know why.

Because, besides the screen the thing that uses most power in a phone is not the processor but actually the Radio.

Placing the phone down means the radio is facing up and looking directly at the signal towers.

This could mean your phone locks on to a stable signal. If it doesn't your phone constantly turns off, on, searches, locks, and retries for cell signal.

I played DeadTrigger on my NOTE on 1.4GHz cpu and 300MHz gpu speed. Brightness set at 40%. I played it on a flight with Airplane mode for 40mins, it consumed 14% battery (83->69%).

While on the bus, after the flight, I continued the game without Airplane mode. I played it for 20-25mins. It dropped a whopping 24% (61->37%). Nearly triple the consumption.