Made up numbers as an example:
Say you see something on the shelf with a $5 tag, and you know the sales tax is 10% in this area. You would calculate in your head that the price is a total of $5.50.
When you get to the register, the item rings up for $6, so with tax, you pay $6.60. This isn't a "computer error" or a "price change they missed", they are doing this on purpose to make money and hoping people don't notice. If you're only buying one item, you'll see it easier, but if you by several items and your total is off couple dollars you might not notice...
On top of that, there's typically one employee in the whole damn store, so if you do tell them something is wrong now you're the Karen holding up the line over "just a dollar" while they have to go to the back of the store and verify what the shelf says.
It's a combination of illegal business practices and social engineering done for profit by a major corporate entity, and I hope they burn for it.