The player who presents the best package of physical abilities, technique, and can put on a clean performance is going to play over someone whose total package isn't quite as good. Not everyone can be the complete package the likes of Quenton Nelson, which is why the best overall guys are going to play, even if they're less adept in certain areas.
For example, Ryan Harris was a starter in 2003 as a true freshman, and to say that he was 255 lbs was actually being a bit generous at the time (back when most college linemen were around 290-300). Nevertheless, his technique and anticipation were so good even as a true freshman, that he leapfrogged a lot of Bob Davie's guys (who weren't chopped liver), who were bigger, stronger, and more experienced.
Yes, he got overrun by some of the more powerful ends and tackles, but still held his own very well.
On the other hand, Trevor Ruhland had a starting spot for a while, since his technique was good, but he was playing on two bad knees that most of us wouldn't even be able to walk on, and when Josh Lugg stepped in and took over, Ruhland wasn't coming back, since he knew his time was over.
Going back to the Holtz era, everyone thought that Jordan Halter would have seized a starting spot on the line, since he was easily the best athlete among all of the offensive linemen, while also having a big frame, very good footspeed, and quite a lot of strength, but he was beaten out by Lindsay Knapp (20 lbs lighter than Halter), who had much better technique.
Or on the defensive side, Bryant Young was barely 255 lbs as a true sophomore, having switched over from linebacker in the spring of 1991. After George Williams got kicked off the team, everyone thought that it would be Junior Bryant or Troy Ridgely taking over that nose tackle spot, but Bryant Young was easily the best man for the job, even if he were badly underweight for the position.