League of Legends Mastery: Weapon Expertise

the differnece is that first they take away the flat penetration and then apply the percentege reductoin
if that wa'nt what you wondered about then maybe it is that weapon expertise takes away a % of the enemys armor when you attack to decide your damage against them
100 armor
attack them with weapon expertise it is 100(armor)/10 so it takes away 10 armor
sunder takes away 2/4/6 armor no matter how much armor they have

whats the difference with sunder and weapon expertise?(in masteries)

crazy op move

Wow 6 ArP then another 10% +runes? True damage at lvl 1 much?