4-5 Star O-Linemen are the toughest to find of any position, and they take some time to develop in college. So much of it is based on potential (no stats like skill positions), because many of these guys have only recently grown into that frame.
Recruiters see a 6'5" guy with a great reach, maybe 3-bills in HS and lots of teams will take a chance. They figure they get him on the weights and his upper body strength will improve, they get on his foot work, and just maybe... Doesn't matter if he hasn't shown the intensity or killer instinct like is important for the D-Line guys; he looks good on HS field. Then when he get's to camp he doesn't look so good going against 330 pounders who can dunk a basketball.
I played on a small college O-line and the other OT was 6'8" 285 - he played only one year of HS ball, started in college the next. We ran a quick toss, and he could get out in front (4.9 40) and he rode that to being a small college All American. He had a shot with the Chiefs, but didn't have the upper body strength. It's all relative.