I'm making an assumption here, but:
The kerashim of the Mishkan were 1 1/2 amos wide and 1 amah deep. According to the braisa that holds that there were three middle rods, the nominal length that the rods held in place was 72 amos (30+30+12) [I have no proof, but I'm assuming the corners were held by the western rod, not the north and south ones].
However, according to Rabbi Yehuda (Shabbos 98b), the depth of the kerashim were gradually shaved from an amah to a point on top (k'etzba). In order to compensate for the jut at the two corner planks, these corner planks were shaven to meet the shape of the north and south walls- from 0 at the bottom to an amah at the top. The center would have a 1/2 amah shaved off on each side. So the middle rod covered an area of 30+30+11=71.
(Though it doesn't seem like the actual rods were that long, or they would have to be shaven, too, so it wouldn't be proud of the keresh, and that isn't mashma.)