It's a confirmed bug by blizzard:
http://forums.worldofwarcraft.com/th...geNo=1&sid=1#1
Hortus' post
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Improved Concentration Aura not applying a full 30% reduction to spell locks is a known issue and will be fixed in a future patch.
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I'd hold off respecing if you're not playing till the weekend or something as I've heard they might be hotfixing this issue, as there are other bugs such as Misdirect occasionally giving the threat to the hunter.