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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. so why I keep witnessing for example in the Boeing 737 800 PFD that at low altitude let’s say your stalling speed is around 140KIAS and at cruise level stalling speed is approximately 220KIAS (random number) Why is that ? Doesn’t stalling speed must to be the same at all altitudes? Sorry for the basic English .