Bamboo is a grass, it doesn't layer bark. It's one and done. The internode distance is pretty much fixed once it hardens. The bamboo cells inflate and harden. So the graphs make a reasonable amount of sense.
The wall thickness is a function of time to harden, and time to add extra stuff to the wall. Segments close to the ground have simply longer time, and likely hardening begets hardening.
The internode distance is interesting in that there is a natural point at which the bamboo stops elongating and begins to start shortening (again in a negative exponential). My understanding is that bamboo does start to keep "leaves" that sprout from the nodes, to gather light and energy. I wonder if that is the point that the leaves start, and hence it is a mixture of both time (less time = shorter) and investment in energy (effort to grow sun-catching leaves).
Some interesting followups would be in the natural habitat, where does the typical canopy start?