Apple most likely checks each panel thouroughly in order to make sure that it is without flaws. They are all about appearance, just look at the stuff they produce. On the other hand Dell most likely gets all the panels they can, produce LCDs on them, and get them to market. They make their money by undercutting in price but shipping a larger volume. The return ratio to purchase ratio probably heavily favors the purchase to their benefit. You may hear about a lot of people returning the panels here, but I bet the vast majority of people who do buy them are uninformed about things such as backlight bleeding and dead/stuck pixels. As such they keep the LCDs also to Dell's benefit.