That's the trade off of self checkout scales that try to match what you're buying compared to those that don't seem to care at all. At some stores, it's overly sensitive, and you constantly need attention to get past the system thinking you're trying to steal, and even adding a paper bag is enough to cause problems. At other it doesn't seem like it cares at all, and two items can be on at the same time without it noticing. Maybe they track it all and just review cameras later?
Produce charged by the item is probably hard to account for, but three times the normal weight seems like something that might be safe to trigger on.
I really really hate the weighing self-checkout things that are so prevalent in the UK (and I guess US?). They require a special artform of weighing the things just right, putting it one by one in the right basket etc, which is slower than going through a normal check-out.
In contrast one of the bigger chains in The Netherlands has a self-checkout system where you get a small hand scanner to put in a holder on your cart, it doesn't make you weigh anything, just show it the barcode before dropping things in your cart (or easier: put your bag in the cart, put everything straight in the bag). At the exit, drop the scanner thing in a charger, hold your phone in front of the checkout screen, Apple pay, done, straight to the car with your stuff already in your bag.
Their UX guys understand how to optimize. The original version made you press a few buttons to select things like "card payment" (still takes only 30 seconds vs the Tesco several minutes of weighing things), a few years ago they made it default to card payment, so you don't have to press any buttons if you use Apple pay or similar.
I guess only Amazon has an even better thing with their "walk straight our" shop. But it has the same optimization idea: Make it as easy as possible for your customers to buy things.
Yeah, around here Target seems to be easiest. They don't care if you use the scale at all. You can just scan and put into a bag someone with you is holding, or whatever, it never needs to touch the scale. Each self-checkout does have a little security screen above it through that's about 4"x6" that shows you and says you're being recorded, so my guess is that it along with the fact most people are paying with credit makes purposeful fraud fairly low, or at least low enough to be worth while.
Produce charged by the item is probably hard to account for, but three times the normal weight seems like something that might be safe to trigger on.