I've only ever noticed this in one circumstance. I was a Gladiator, shield equipped, guard in use. I'd get "Block! ... Shield skill increased..", but occasionally I'd see a "Block! ... Sword skill increased..". I got confused as to how I was gaining sword skills for blocking, but then started paying attention to the animations. While you have your shield up you will normally block with the shield, but you can and will sometimes block with your weapon.
A few days later I had forgotten about this and was trying to get my Thaumaturge to 10 when I was momentarily confused by a "Block! ... Thaumaturge skill increased.."