That's not how self checkout systems normally work - they build up their own internal databases of average weights over time. This has a number of benefits - it saves time and money for the store (you don't have to pre-program the machines with weight values) and it also allows for varying tolerances by item.

Nonetheless the point stands - the original barcode can be used to match up against the internal database of average weights.

Is that why new self checkout systems are immensely frustrating, but after a few months they're fine to use? I'd assumed that the tolerances were lowered because too many customers were getting so frustrated that they were refusing to use them.

Yes, but they have to recalibrate them every so often and they get overly sensitive again.

Problem is, every time it beeps at you and the cashier overrides it, it averages your item's reported weight with the ones its seen before. So if a cashier is overriding it all the time (as tends to happen with constantly beeping things) the weights drift off, and the whole thing is quite useless.

My guess is that the red digit encodes the reason for the discount.