XtendScott means padding. Margins are not taken into account for element dimensions.
Also , note you floated an element while absolutely positioning it. It's probably not a problem, but realize it's not good code. Following the standards, you may not get what you are intending, since an element cannot be floated and positioned at the same time.
I did not modify the padding. The top margin of the BODY tag is what I found to be pushing the columns beyond the actual height of the visible browser area. Setting the margin:0; or margin-top:0; eliminated the issue.
"Margins are not taken into account for element dimensiions." - That is my understanding also, so 100% height would not subtract any margins on the element from its surrounding container. Thus the element height would be equal to the height of the container and would still be affected by any margins.