Ideally the cylinder should be placed close enough to the trap door to not restrict access and to allow work on the thermostat/element/pressure control/pressure relief/vacuum breakers etc. while standing on a ladder with only the top half of your torso through the trapdoor.

But it seems it is a standing joke by plumbers that they do everything arse about face, even to mounting urinals so high that you have to pee upwards to get most of your stream inside.

On the other hand most new installations are made before the ceilings are installed and sometimes the carpenter places the trapdoor somewhere else than the position that makes most sense.