[ossiblue]

What I have found, on occasion, is that a 4-foot level does not always read true on a vertical jamb that is over six feet long, especially on pre-hung doors where the jamb is relatively thin. You might have a bit of a wave in the jamb that may be vertical where you place your level, but is actually out of plumb if you move the level down (or up) to the other end. In you case, if you placed the level at the top and got a true reading for the hinge side jamb but didn't move the level to the base to confirm if the lower part of the jamb is vertical, it could result in the door being a bit out of square since the lower hinge may be "kicked in" a bit, lowering the opposite, upper corner of the door, allowing a small gap. Dad911's method is a good way to ensure a square fitting door.