Both objects are with zero position but you cannot add slab object on the ground like barrier.

The slab is set at zero and can't go any lower. Non-concrete objects by default don't get placed exactly at their set height, they are 'dropped' from a bit higher and then placed where they hit the ground.

I've seen blue concrete at level 0 used as water there... not as good as Eric's version would be, but I knew what the author meant

The 'lake' isn't filled with water because it isn't a lake, just a drainage area, I believe.