As you can see here : http://imgur.com/Kx60Xta
The center point is not the same.
Explication :
It's due to the fact that the middle of the block is z = .5 and x = .5 and not an integer.
So the command with the coordinates ---> the coordinates get changed to the middle of the block.
Whereas with the relatives coordinates ----> just using integers as coordinates, so the middle of the range of the spreadplayer command is a coner of the block.