A: This is probably due to low diffusion of surfaces in the room, and 1-dimentional reflection paths. The decay curve will not be linear but will have a steeper curve in the beginning and a less steep curve for the later reflections. With nonlinearity in the decay curve a larger number of reflections is necessary to simulate the reverberation time. also coupled rooms with non linear decay curves should have a large number of rays in the room setup. These are good examples of a volume, where a simple Sabine calculation will not be enough to calculate reverberation time.
It can be a help in cases with nonlinear decays to look at the Global Estimate, to analyze how many rays are necessary to define the decay curve (The Global Estimate should a least go down to – 35 dB at all frequencies).