.....Blasters have the same 100% chance to "penetrate" (ie get through the target's defenses and cause some sort of damage) as spell-based damage such as Ice Bolt. And, as others have said, there's no resistance to energy damage as opposed to the more common cold, fire etc resistances, so your Blaster will pound-for-pound deliver greater bang for buck, in that 3d4+12 of Blaster damage will always cause 3d4+12 of damage, as opposed to 3d4+12 of spell-based damage which has a chance to be reduced by resistance.
But there does seem to be some sort of grey zone around the edge of the sprite which isn't part of the target, and attacks directed at this grey zone may miss. What I think you're seeing is that spells with an individual enemy target must be properly targeted in the first place: you can't cast them on thin air, and the game will only "accept" the spell target if you've got it correctly locked on in the first place. Whereas physical attacks can be made (pressing A or mouse button) without needing to "properly" targeted. This doesn't seem to affect melee attacks, which also always seem to properly find their way to the target, but missile attacks (which the Blaster effectively is) can miss if they're not sufficiently centred on the target.
So in a way, yes, you're correct in that the part of the target you focus your attack on can affect things, but it affects whether the Blaster will hit at all, not the amount of damage. Try this next time: walk up close to a large target in real time and keep the A button pressed: you should see an almost continuous stream of Blaster damage, all of which will hit because the attack is sufficiently centred on the target. Then find a smaller target - human sized of smaller, and stand some distance away, and (again in real time) keep the A button pressed while you're running around or moving the mouse around. You should see more of a strafing effect with many of the Blaster bolts flying past the target and not hitting. Same with bows. The reason you don't see this with spells is that the game stops for you to select the target - even in real time - and won't "accept" the target unless you've got it close enough to hit in the first place.
Hope that makes sense.