Have you tried taking readings from the other side/direction? Meaning switch ends and see if it mirrors the readings.
What about running it against one of your other straight edges and see if you get the same variance?
Also, since you have FEA capability, have you ran a FEA sim where you add a specific weight to the straight edge and see if the simulation matches measured?
Pittsburgh would be a bit of a drive. BTW: you a product of Carnegie? Me thinks your a bit over the edge doing this in your home shop
and I mean that in a very positive way....