Why do people call a decrease of 40% to 20% a decrease of 20% instead of 50%?

[Dennis S]

I had the opposite experience rz had at Dillard's. Some $100 shoes were 50/25 off and they insisted they were $25. I argued but they (the clerk and his boss) insisted. Then they showed me another pair in my size and asked if I wanted them, too. I said "sure."