First of all, I think you're confusing actual physical "cores" with Hyperthreaded virtual
cores. You are correct in that Reason uses the FPU of every physical
core. As discussed at length and according to Propellerhead software, Reason uses HT (virtual cores) for things such as GUI draw, high-quality stretch and resample, and disc i/o. Were you performing any of these when you took your screenshot?