I consider solo tackles a good indicator of Tackle ability. That said, I don't see where Wagner should have a tackle rating of at least 10 points higher than Buchanan. Buchanan is faster than Wagner and excellent at tackling. You can look up Buchanan's Bio and see what he did in College as a safety.
I have watched Arizona games and I've watched Seahawk games, and I don't see Wagner's tackle ability 10 points higher than Buchanan. I don't see Wagner's tackle ability higher than Buchanan period. The only reason Buchanan gets the short end in this game is because he is rated Normal Development--and in this game, Development means an awful lot. In my opinion, Buchanan is way underrated in both tackle rating and Development.
Buchanan even has more sacks than Wagner: 3 to 0.5--which is amazing since he is a safety playing MLB.