It might not have anything to do with the upper receiver. I noticed this phenomenon with the DD RIS II rails on all of my rifles, especially the longer rails. It has to do with how tight the 4 screws that attached the bottom portion of the rail are. The rails, in my case, will always line up perfectly center with the barrel before I install the bottom rail. But as I tighten the 4 screws, one at a time, I did notice that it will pull the entire rail left and right depending on what which screw is being tightened at the time.
Try this, loosely tighten all 4 screws, the proceed to torque just one screw next, you will notice that the entire will pull closer toward the barrel on the side of that screw. So to pull the rail to the other side, proceed to torque the screw on that side next, the rail will line up with the barrel again. Repeat this process until you have all 4 screws properly tightened and the rail and barrel both lined up.