A childhood belief about money

As a kid, I could not understand why the cashiers in supermarkets would give change to you when you'd pay. The only explanation I could figure out was: you're to pay $100, and you only have $90 in your pocket, so the cashier gives you the missing $10. But I thought this was weird, though, because any dishonest person would just pretend having only $20 in order to get $80.