Are they the same genetics? Some plants just eat less.

I second this opinion. It looks like the early stages of nutrient burn. It could be as simple as not mixing the nutrient solution enough so the bottom of the barrel is hotter than the top. My first suggestion to every grower is always less is more. I like to use super soil up until flower for that reason specifically, and then just use supplemental nutrients twice weekly until flush. Unless you are growing commercial crops on giant plants or are in a CO2 supplemented environment then it is unlikely the plants are getting large enough to require large amounts of supplementation.

maybe the heat, everyone shaded the same all day?

See my above comment, but I will also say that Cherry Pie is notoriously finicky in my experience. I dumped that strain after two rounds because A: I didn't like the genetic that much and B: it always got locked out while the rest of my plants were going gangbusters. Blue Dream can withstand a nuclear holocaust though. I swear you could piss in the pots and get 2 pounds off a plant.