Well, as said they use the correct dice for the weapon but for some spells they use weighted (even not for all). The reason they do this for some spells is (a) probably a balancing thing and (b) as a lot of small dices like 1d3 flaten out the result but you may actualy want to see a higher distribution.
So the first thing where I am totaly on track with Borror0 that they maybe should first add the correct dices they use in the description at all instead of giving a false impression, independent of the discussion about the ranges. But that is just a good wish and my opinion...