^^^ That's the answer...
If the barrel isn't drilled right...
So, the barrel extension has to go on the barrel FIRST, because of the feed ramps in the barrel extension - they center everything. That thing gets torqued on at 175lb/ft, to the barrel. THEN the extension (and feed ramps) are aligned, and that's the CENTER of the barrel, and the rest of the machining can take place. After that, the gas port gets drilled ON CENTER with those...
It's not gonna be the MagPul rear sight that's off - but if it is, MagPul will want it back to check it out, and send you a new one before you even take that one off your rifle...
However, in that pic above, that rail is NOT centered on that upper receiver... Does the rail have alignment pins to center it? WITH the barrel nut?That can throw the whole thing off, right there...
Might be that all is machined right, but the barrel/barrel nut is not properly installed - torque dictates that if it's gonna be off, then it's gonna be off in the direction that HIS is off... I'm just sayin'...
Look at it close...