They need to make sure their scanning system is correct. There have been a few occasions when the items are not correct, forcing a close choice, or they do not allow you to input a quantity.
Minor glitches but still....
You are right. Those self checkouts have a lot of errors. I buy a lot of clearance items and they wring up wrong about a third of the time. I then have to get the girl to go through a long process to correct it.
At the regular checkout I just tell the girl that it rang up wrong, she looks at the sticker, removes the item then reenters it.