I'm not a pro but I know for a fact it's to do with the climb profile of the aircraft. As the aircraft climbs it's usually on autopilot (VNAV or FLCH) as you climb above 10,000ft you accelerate to optimum climb speed, may be 300kts or 320 or whatever all depends on the type of aircraft, but anyway you maintain that speed until the aircraft reaches its mach cruising speed. As the temp drops so does the speed of sound so when the aircraft hits, for example, M.80 it will continue climbing to cruise alt but wil also start slowing down ie going from 320kts IAS down to maybe 290kts IAS at its cruising alt since the temp drops with altitude. However the groundspeed won't slow much because as the aircraft climbs it climbs into increasingly thinner air. The other thing is as the aircraft climbs its TAS (true air speed) increases due to thinning air and errors in the pitot tube so the max IAS (indicated airspeed) drops. So perhaps the aircraft has a max IAS of 350 kts at 1000ft but at 35,000ft it may only have a max IAS of 300kts. You might hit M.80 at 310kts IAS at say 23,000ft but by the time you reach 35,000ft 310kts IAS is beyond limits and M.80 will be perhaps 295kts. I realise this is long winded but it has to be to explain it properly