Hello papa600,
mainly, because the IAS is not anymore a actuel figure of the A/C speed..
for exemple, you could fly in a climb at a constant IAS..but the MN is constantly increasing...furthermore the speed limitations of the A/C will be based on MN..let's say above FL300
For the instrument..I confess it's quite "long time ago" but I still remember that's quite simple in term of technology...Now using air data computer since a decade (at least)..everything is computed according to the different physic rules..starting with: speed of sound = 39 X square root T° (absolute in ° kelvin) and MN = TAS / Local speed of sound...finally this is simple math's..and glad to see this done by a silent pax...
Other advises will come up certainly..
Good evening