This is where price discrimination comes in, and it is staggeringly effective: coupons, sales, discounts, etc etc. The great thing about fixed prices is that there are so many to choose from!
They almost certainly get more money.
They are selling the budget chickens to the budget customers at a profit. They are getting the same chickens at a larger profit to other people.
Think of it this way: a rich man is willing to pay up to $10.00 for the chicken breast that his family needs for dinner, but his blue-collar cousin can only afford $6.00 at the most. Had the grocery store only created one line with the price of $8.00, their revenue from these two consumers would be only the rich man's sale, 8 dollars. If, on the other hand, the store had two lines, one for $5 and one for $10, they can make 15 dollars from these two shoppers!
Hope that clears it up.