Somebody explain this one to me.

A word problem:

I want a vending machine item that costs 90 cents. I put a one dollar bill in, and the machine replies that it would like me to use exact change. This irks me, so irrationally I put in an additional dime and select the same 90 cent item. The machine cheerfully vends my item, and gives me change: two dimes. Presumably these are my extra dime, and the dime it should have given me as change in the first place.

Can anyone explain this behavior?