One thing we need to remember is that in training camp/pre-season guys are being tried at different spots along the line, taking reps against different defenders every day, lining up against unknown players from other teams, and focusing on fundamentals. Whereas during the season...the coaches have guys watch tape from Wednesday-Saturday on 2-3 guys that they will likely line up against, make sure they practice specific blocking techniques for said opponents, and make sure to scheme teammates on both sides to help out (e.g. Russ Hochstein vs. Panthers DL in Superbowl XXXVIII).
It is how Connor McDermott played fairly well down the stretch last year when pressed into duty, IMO.