"Overscan" is built in to all of the old tube-type TV sets. Basically, this means that the edges of the picture are "cut off".
The only way to make sure that you see the entire picture, regardless of "overscan", is to zoom out on your pictures (or edit your pictures in an external editor) such that they are zoomed out by 10-15%.
Look in the title editor -- see the rectangle? That is considered the "safe area", the area in which you are (usually) able to see on any given TV set, unless it's completely out of kilter.
Some discussion on "why overscan exists" and "what can be done to detect problems with overscan", etc. can be found in this thread.
BittMann