because void staff is against tanks O.O
so unless you play vs tanks its best to go liandry + rylai.
UNLESS you build your mastery tree specific on using voidstaff.
sorc shoes and liandry don't match well with voidstaff at all,
the formula to calculate damage done goes as followed :
base damage times OLD armor/mr divided through a hundred plus NEW armor/mr = damage done.
lets say you have 30 flat pen and 200 ap and they have 100 mr
first without pen = 200 X (100 / 100 + 100) = 100 damage
with 30 pen = 200 X (100 / 100 + (100 - 30) 70 = 118 damage
now lets take 35% penetration
100 / 100 * 35% = 65 penetration thats :
200 X (100 / 100 + (100 - 65) 35 = 148 damage
now lets combine them :
100 - 30 flat pen = 70 which means 70 - 35% = 45,5 penetration
200 X (100 / 100 + (100 - 45,5) 54,5 = 129,45 damage
noticed how flat pen ruins percentage pen ?
use percentage penetration against 90+ mr anything below that level of magic resist,
will be torn better by flat pen, unless you can exceed the 35% with runes O.O.
most players will keep their champion around 80 mr to avoid void staff, only real mr tanks will go over 80 mr, truth is void staff just doesn't give the same stats as liandry does with ap, and it can double the health damage AND slow down champions with rylai.
though liandry/sorc shoes/rylai are less effective vs squishies than they are vs health tanks due to the health burn :P but it still packs a heavy punch vs low mr champions.
so if tanks/champions are <90 mr go liandry sorc and rylai
if they have 90> mr go void staff ^^
just keep your masteries in mind .-.