It's not pettiness. If you went to a shop and asked for 20 fags, and, in front of you, the person behind the counter took one out and gave you 19, would you complain? Of course you would.
If someone serves me a short pint, in a Sams or elsewhere, I want it topping up, because that is what I'm paying for. And God help anyone who refuses. I'll shop them to weights and measures pronto.