Bless them, this is because the FAA documentation says compressibility need not be considered below 200KT (one assumes CAS but they don't specify) and 10,000ft (missed that bit out) and the CRP-5 and its RAF predecessors did not consider compressibility below 300KT TAS. The response is an awkward attempt to reconcile the two given speeds, but sadly misses the point that compressibility is not applied in the conversion between Mach number and TAS. Really.