There are images around the web of 6-06 Joeckel and 6-05 Matthews standing shoulder to shoulder. Matthews is a good two inches shorter than Joeckel and doesn't seem to have arms that are disproportionately long, where that would compensate for a shorter build. These are off-the-field images for publications/media guides, so could be poor photoshop composites.
If he were 6-03 3/4, still an OT?
Haven't seen many sources that had Matthews rated low; Many had him up in the top 5-10. If that were accurate, how could he improve his stock for next year. There are already better LT prospects for 2014. Maybe he didn't get that kind of response from the NFL scouts.