I had written to RB and here is their reply...
Our customer service department has relayed the following; "Our foam board thickness is a nominal measurement. There may be slight variations from carton to carton."
Based on personal crafting experience, I have noticed that how the boards are stored, and even the temperature where they are stored can sometimes affect them. For instance, I try to keep all of my boards pressed between two solid objects. The temperatures (and humidity for that matter) in my crafting area fluctuate and making sure there isn't any wiggle room between the boards keeps them more uniform. Unfortunately this also applies to how they are stored between our production line and when they hit the shelves. Dollar Tree allows people to order a full case through their website. I would expect the boards to be more uniform within a single case, rather than matching single boards off the shelf.