Most of the treated wood is only treated to a point into the wood and not all of the way thru. This of course varies with the amount of treatment applied and can vary from 1/2″ on up. It is possible for a .60 2×4 to be treated all of the way to the center while a 6×6 post may not be. When the wood is cut this will be noticed as a lighter color and in order to make the wood last the longest the ends of the wood need to have a treatment brushed on.