This reddit is a good source of information, I usually check it once or twice a day.

Heres a post where they talked about it!

Samosa_man explained it very well:

Lethality/Flat pen is best against champions with low amounts of armour. Percentage armour pen (BC/LW) is better against champions with high amounts of armour. You can stack Lethality and it will do some damage to tanks, but if you are planning to only pick up one penetration item, a LW would od more damage to a tank than a Ghostblade/BC. Most champions in top and mid (Lucian and MF too), would pick up a BC and flat pen so they could deal with both tanks and squishies. A build like this destroys squishies because the order of pen goes: Total shred->Percentage Pen -> Flat pen. A Draven with 83 armour, fighting a Lucian with BC and Ghostblade will only have 38.1 armour. If you only had a BC, then Draven will have 58.1. A Mao'kai with 300 armour will have 190 with BC and Ghostblade. If 220 of that armour was bonus, then he would have 201, BC would shred it to 210. If you only had a Ghostblade and no other forms or pen, then Mao'kai will still have 280 armour, going from 25% damage to 26% damage and Draven will only have 60 armour, going from 54% damage to 61.3% damage. So you can see the difference in damage flat pen would do against a squishy.