I don't get it either. For a bill of £4.90, you were already owed £5.10 after giving the tenner... How does throwing another 20p into the mix simplify anything

I don't get it either. For a bill of £4.90, you were already owed £5.10 after giving the tenner... How does throwing another 20p into the mix simplify anything? Now they owe you £5.30