In the game the void is the same as the indestructible walls from CW1. But the void is basically just nothingness. Shouldn't the creeper that flows to the void just fall in and be destroyed?
I think of it more as space - and creeper drifting in space can sometimes fall back to 'earth'.
Remember, from a gameplay point of view, void is like 500 height terrain - so massive quantities of creeper can overflow 'onto' void and spill across the surface.