From what I can tell the variations are both due to different manufacturers AND sample variability within a single manufacturer. White LEDs are commonly produced by taking a blue LED and coating it with phosphor, such as a yellow phosphor. There is some variability in this process especially in the high yield low cost area. This seems to account for variability amongst screens which appear to me to come from the same manufacturer but range from cool to quite yellow. This variability in manufacturing also imposes unequal luminosity from issues such as heat loss at varying degrees and variability in the LEDs used. So, in sum, I have seen some dim, yellowish screens that look awful, cool screens with uneven backlighting but some parts bright, and a mix of those things. The one I have now is kind of neutral and very even backlighting and is pretty bright.
There is another variant I've seen that is consistently bright and a very neutral white with great color but a characteristic falloff in luminosity on the left longwise edge. It's identical in all four examples I've seen. Weird. Probably a different manufacturer. Aside from that issue, IMO these are the best looking screens. They do tend to have some bleed but the ones I've seen it wasn't severe.
I am not personally a fan of the cool temp screens as the color isn't accurate. There are bright neutral-to-warmish screens out there. I think the yellow ones may be cases where there the phosphor costing was poorly applied but I don't know.
