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Why does indicated stall speed change?

Aviation confusing me... I’ve read that stall speed doesn’t change (IAS) no matter what altitude you’re flying - of course under specific conditions ISA, 1G level flight, no wind, gross weight etc. then why I keep witnessing for example in the Boeing 737-800 PFD (primary flight display, intend to the barber pole) that at low altitude let’s say your stalling speed is around 140CAS when at cruise level stalling speed is way above - approximately 220CAS (random number) Why is that ? Doesn’t stalling speed must to be the same at all altitudes?