What makes one modern barrel more accurate than another? Put a good shooter behind two barrels, why does one shoot a minute, and the other shoots 3 minutes.

Case in point, I have a Colt SOCOM upper that easily shoots a minute, and a BCM SOCOM upper than shoots 3-4 minutes at the same distance, with the same ammunition, under the same conditions.

In the case of both of those barrels in particular, I assume we're talking about the same materials used, the same manufacturing process, etc., so where's the difference?