It should but as you deducted yourself, that is not the case. I suspect they force the profit margins of vegetables way up somehow. I'm sure they've done a lot of research and experiments and concluded that:

* People are willing to pay x for a pepper

* People do not consume significantly more peppers if the price is lowered

* People are willing to pay y for a kg of chicken

* People buy significantly less chicken if the price goes up much higher

TL;DR it's about price vs volume vs profit margins, and I'm sure they continuously hover around an optimum for these products.