When you have a bucket of water, you place it on a slab (I placed it on a wooden slab) and the water does not appear but it then shows the water at the nearest whole block. Say you have a slab floor, you place water and the water appears on the outside. The source is there but it doesn't show, same with flowing water. It doesn't show until a whole block is in the way. Also, it goes through the slab if there is space underneath.