As far as I know, all of the walnut stocks used by Springfield come from Boyd's. There will be some variation due to the nature of wood, but I'll wager what you are really seeing is a difference in stock finishes. These stocks come from Springfield really dry and in dire need of some oil. Some folks use Boiled Linseed Oil, and others use Tung Oil. Some others may use a mystical concoction. Boiled Linseed Oil tends to darken up over time, where Tung Oil typically does not have such a dramatic change.
Of course, if you are comparing Springfield's catalog photos of the rifles, I wouldn't put much stock in the visual differences.