I notice it too--it doesn't have to be a Bard, and it doesn't have to be melee, in fact you can see it better with arrows: Sometimes the real Mage will get hit while his illusions are unharmed.
You know how Thieves can dispel illusions, provided they've put some points into Detect Illusion and you turn their Find Traps on? Well, I've heard that characters with high INT (such as your Blade) can do the same thing, albeit on a smaller scale. They just naturally figure out which Mage is the real one.
This isn't the only instance of things going on in the background: In PnP Dungeons & Dragons, Thieves and Bards have the skill of Climb Walls. Believe it or not, this is actually implemented in ToB: There are some ladders in Amkethran, and if your DEX is too low or your encumbrance is too high, your characters will fall on their ass and get hurt a bit. The game is actually rolling and checking the roll against each character's Climb Walls ability, even though you didn't even know they had one.
I wish I could sound so educated about the Stoneskin part of your question, though.
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Volothamp's Comeuppance
Everything you ever needed to know about the entire Baldur's Gate series......except spoilers.
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