Xbows were made to penetrate heavier armors, but apparently it isn't possible to implement properly in warband. Or the only way to make it penetrate armors is to give it so ridicilously high damage that is exceeds the soak value of armor. Sadly this also makes it the awp of this game, one shotting everything with 50 or less armor, even if it hits only in the toe.
Imho it should rather do a more fixed amount of damage (like 40 or so), by pretty much ignoring the armor. Would be nice if you could set different soak values to armors on weapon basis, not just damage type.