Since daggers can be used with a thrusting or slashing motion, it may be that they have a small unlisted component of slashing damage. High armour class opponents resist that weak attack completely, but lower class ones do not.
If so, they are similar to crossbows, which do a little damage based on the weapon type (crushing for normal crossbows) in addition to the damage type determined by the arrow used.
The slashing damage may be a bonus from your stats or the weapon expertise skill (ie thrust in to do most of the damage, and then flick or twist the dagger pulling it out for a little extra). After saving the game, you may want to try unlearning your one handed piercing skill, and see if you still get slashing damage.
It could also be a bug, where you are getting a bonus from a slashing expertise skill that you shouldn't. Try either boosting the damage portion or unlearning other weapon expertise skills, to see if that makes a difference.
Lady_Rain;
The numbers for 'vs. piercing/crushing/slashing' are bonuses to your armour against those specific damage types. I don't think they effect your attacks at all.
