Subject: Why do I need less water to boil more eggs?

My electric egg boiler consists of a heated metal plate, covered by a 
ventilated perspex dome. A measured amount of water is poured on the plate 
(whilst still cold) and the resultant water vapour (steam) cooks the eggs(up 
to 7), which sit on a perforated plastic tray inside the dome, above the water 
level.
After a few minutes a buzzer goes and you take the eggs out.
There is a calibrated water measurer to enable you to pour the correct amount 
of water on the plate. What I don't understand is why you need less water the 
more eggs you have.

Re: Why do I need less water to boil more eggs?