Well if I were going to speculate, I'd first take into consideration the differences from one strain to another. I've grown a lot of hydro and seen some big differences from one to the next in the main stalk when I chop them down, some being almost completely hollow and some being quite dense.
We also know from experience that hydro plants tend to grow faster than soil, so the soil has a lot more time to grow internally while the stems remain shorter.
I also wonder at the differences in the availability of micronutrients between organic and synthetic fertilizer - most of the organics have everything always available in the soil and the plant takes up whatever it needs, while with hydro you're pretty much force-feeding a blend that can sometimes skimp on a micro here and there. I've seen some talk on supplementing with silicon in hydro which I don't personally know if it effects the pith growth, but I wouldn't be surprised. A lot may come down to what regiment you're using in the hydro grows, in that regard.