the more rock is hard - the more lp you should gain.
for example the standart LP gain is 18 for all.
Since the hardness is 200 (20 strenght*10 quality stone axe) - Lp gains changes to 22
and so there should be a proportional LP grow.
however i guess the pick is 54 LP gain (or more) For like 4000 hardness (100 strenght*40q tool) something like that.
Also it may depend on Ore quality, or stone quality. It even may be a new "Mining skill" like a carpentry, which will boost ore gaining chance. It's only the suggestions, so only developers will chose

(My english is sucks, yeye.)