(1)At supermarkets, particularly for fruit: red plums and black plums cost the
same/lb, xxx apples and yyy apples cost the
same/lb etc. each have their unique bar and product code on them, but sometimes they get co-mingled. You get to the cashier who weighs them, but somehow she enters the red plum code wrong [in one instance they had to call a fruit person]--if I had, say, one red plum and one black plum, weighing them as ONE item, it would still cost the same, yet they are spending lots of time trying get it EXACTLY right.
(2)I was in a McDonalds and ordered a soft-serve cone, but they ran out of cones, I suggested they put the same amount in a CUP--at that particular MD it was too difficult an operation to deal with [I actually prefer it in a cup!]--they couldn't sell it to me.
1) Accurate sales data allows for better ordering and such.
2) Some restaurants have strict audit procedures to ensure staff are not giving away food. ie cups are counted and compared to what is sold.
Most would have a way to accurately ring up the item (ice cream in cup), though. If nothing else, a sundae without topping...?
Anyway, must be a pet peeve of yours, but not anything I get worked up about.