Why Does The 777 Overspeed at 330Knots?

[QF6024]

I always have my gauges reqd the IAS on the gauge. As alluded to the the higher you fly the less dense teh air and you will notice the red line on the
gauge gradually crawl back. so for sake of argument whereas at 30000 ft the overspeed will kick in at 320kias at 40000 the overspeed will kick in at
about 280 kias and continue to decrease the higher you go until the fall is at the critical level where the air canot support the aircraft in normal
flight without a stall being incurred( I think that is gist of it)