It is a known bug with SOA and TOB that affects all non-warrior classes that somehow get specialization in a weapon, and as far as I know is not fixed in any patch. See Baldurdash for description of the problem:
http://members.rogers.com/mrkevvy/buglist.html
Search on that page for the phrase "weapon specialization".
*SPOILER* (very minor, NPC related)
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This problem also affects Haer'Dalis, who is specialized in Short Sword. He also does not get the extra attacks he should be entitled to due to weapon specialization.