A price a penny below a round number is so common that it reads as normal rather than as a decision. It is a decision, and it survives because it does several jobs at once.
People read prices from the left
A shopper scanning a shelf takes in the leading digits first and often stops there, because that is enough to place an item in a rough band.
A price just under a threshold changes the leading digit, so it registers as belonging to a lower band than a price a single unit higher.
The effect weakens when someone compares carefully, but a great deal of shopping is not careful. Most decisions on a supermarket aisle are made in a second or two, on the leading digits alone.
The ending has become a signal in itself
Over decades of consistent use, particular endings have come to mean something to shoppers independently of the arithmetic.
An ending just below a round figure reads as a standard price, while other endings are widely used for reductions and clearance, so the digits carry information about the offer.
Retailers exploit this deliberately, using one ending across a range and reserving another for markdowns, which lets a shopper identify discounted stock without reading a sign.
Round numbers signal something different
Premium goods are frequently priced at clean round figures, because an odd ending suggests that the price has been engineered to look smaller.
A round price implies confidence and simplicity, which suits a product sold on quality rather than on value for money.
The two conventions coexist in the same store, and the ending on a shelf label is one of the quickest indications of how a retailer wants a product to be understood.
The convention has practical origins too
One long-standing explanation is that an awkward price forces a cashier to open the till and give change, creating a record of the sale.
Whether or not that drove the original adoption, it stopped mattering once electronic tills and card payments made the record automatic.
What kept the convention alive afterwards was familiarity: a price that broke the pattern looked wrong to shoppers, and looking wrong is expensive.
It complicates the arithmetic on purpose
Odd endings make mental comparison harder, especially across different pack sizes where the sums stop being tidy.
Unit pricing on shelf labels exists to counter that, restating everything in a common measure so sizes can be compared directly.
The gap between the displayed price and the unit price is where most of the confusion in a weekly shop sits, and reading the smaller figure removes it.