Esso currently has a Pepsi promotion where a customer can buy 2 Pepsi bottles for $3. If you take advantage of the promotion the two items does not ring in at the advertised price. The price differential is minimal. My guess is that Esso is hoping customers would not notice the discrepancy.
If you request the clerk to override the price to the correct price, he was uncooperative.
It looks like a sneaky way of trying to get a few extra bucks given that they probably sell a large volume of the products during the promotional period.