This could be because of changes in the primary skill. For example, if you go from bard to fighter, you may gain an attack simply because of the bonus in close combat (may cross the 50 skill threshold). But that's different from saying that the fighter INHERENTLY gets bonuses to swing/attack by level. I'm not sure one way or the other, but just based on what i've observed, it seems that class doesn't matter that much.
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