While it is certainly possible that the front sight assembly was assembled canted, or the barrel locating pin wasn't set in exactly straight, the usual cause of the condition you've described is that the barrel locating pin got mashed into the left side of it's slot when you assembled and tightened the retaining collar. This canted the front sight just enough to cause the right shooting impact. It only takes a couple of thousanths of being off to significantly alter the point of impact.
Pull the barrel, and take a look at the channel the locating pin slides into. My hunch is that you'll see a depression in the left side of the channel. Whether you do or not, see if there is any sideways play between the pinn and the side of the channel. If there is, you need to put a shim, probably .002-.004 on the left side of the channel, and reinstall the barrel. If there isn't any sideways play, then either the locating pin or the channel are slightly off and you should open up the right side of the channel a couple of thousanths, and then put the shim over on the left side. DO NOT screw with the front sight assembly. It's all controlled by the placement of the barrel in the upper.
Putting an AR together may look easy. Doing it correctly isn't always as simple as some of the people who post here make it sound [:D]