I bought some merchandise with a ticket price of $429. However there was a 15% holiday discount, so I actually paid less than that.

The next day, I needed to exchange the merchandise, so I chose other items with a ticket price of $439, and the same discount. Just $10 more, so after the discount I needed to pay an additional $8.50, right?

Wrong. I had to pay an additional $9.

On checking the bill, I was quickly able to confirm that there was no error in the shop's calculation.

What is the explanation?