Why am I not getting caught for giving extra change at McDonald’s?

I work at mcdonald's and take a few orders a shift at the front counter. I really suck at counting change, so sometimes I give my best guess and just give the customer what I think is the change I'm supposed to give back. I always make sure it's not short. Also sometimes we don't have the correct coins to make a certain amount. I don't want to bug my manager so I give them like 20 extra cents. I've gotten some funny looks, but no one says anything. I close and the manager im closing with always count the tills, and one night my manager was sort of concerned because we came up like $25 short. How have I not gotten caught? Am I just lucky?

Why am I not getting caught for giving extra change at McDonald’s?
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