Yes, it IS frustrating....and to the check-out operator, too! I used to work at Tesco and occasionally had to operate the '10 Items or less' till - and I'd get just as wound up about having to process large quantities of goods as the other customers. But the policy of Tesco - and of other supermarkets - is that they cannot afford to lose even one customer, so turning away someone with a large number of items on the '10 Items or less' till is not allowed, in case they get shirty about it!
Just once....just once, I'd have liked to have said: "Sorry, madam, but you have more than 10 items. Please use one of the other check-outs"!
Mind you, perhaps more irritating was an elderly man further back in the queue, on one occasion, who literally counted out every item that went past my scanner and then said, in a loud voice: "That's 15 items! She's no right to use this check-out!" He was right....but I wish he'd said it to the woman herself...