The correct answer is surprising – becasue it shows how complex an egg reall is :
Air and even water can enter eggs through the shell, it not a fully airtight barrier. This is why the mineral oil works to delay the rotting – it seals the tiny holes that allow air to pass.
Additionally, eggs also have a small pocket of air inside them which the chick would access just before hatching.
Over time, if the egg does not hatch either because it’s not fertile, or because there is something wrongs with the chick, the membrane between the egg contents and the air pocket begins to deteriorate, and the air then assists the eggs decomposition.
Eggs that have gone off will float in water due to the build up of gasses from decomposition where as good eggs sink.