The pump's tubing normally catches air in and out of the pump. At rest, it might reside a bit higher up, but in reality, there isn't a lot of 'space' inside these coolers like you would think just looking at the exterior housing (check out some YouTube or disassembly sites)
At pump operation, the pump housing will still have a little bit of coolant, but also some air, which it cannot displace by pushing it beyond the inlet or outlet of the pump itself - in many situations, this creates an airlock at the pump itself.
Middle left is wrong - air gets trapped in the pump and cannot be moved to the radiator tank, if it isn't there (in the illustration). The buoyancy of air can prevent it from being pushed 'down' into the radiator.
The middle right is correct, the pump does not have to force air down, just laterally, if there is any air. This is a much easier task. Also, the radiator end tank can capture air there and won't be pulled back into the pump over time.