T-grains do have more surface area than cubes or octahedra for equal mass, and that might be part of it.
OTOH, some conventional grain films are also more sensitive, I have heard. This might be due to the fact that almost all of the films today are made of blends of emulsion sizes and this causes them to be sensitive to some development conditions. Each component develops differently and some effort must be made in film design so that all of them react alike so as to make a smooth curve. Any error in processing can emphasize the differences in the blend.