40 yrs was just a random #. Plausible. Depending on site and species could be 20 or 200 just as easily.
For the white bark:
Imagine the "back" is a cylinder attached to the "body" and the "arms" as a separate cylinder attached on the bottom side. What part of those cylinders is most exposed to things rubbing against them (knocking bark off)? Not the underside or top side, but the parts that stick out to the sides the furthest. That is where the bark is exfoliating as expected.
For the white bark:
Imagine the "back" is a cylinder attached to the "body" and the "arms" as a separate cylinder attached on the bottom side. What part of those cylinders is most exposed to things rubbing against them (knocking bark off)? Not the underside or top side, but the parts that stick out to the sides the furthest. That is where the bark is exfoliating as expected.