the resists to mages is the biggest example of a shitty job
statistically, sure you can argue that out of all games played mages were running hp/level seals
but they were running hp/level seals against other mages. Which happen to be the most common class in midlane. in champ select, during select your runes/loadout phase, you would switch to your "I'm battling another mage as a mage" runepage and set your masteries accordingly.
but if you were against a zed or a talon or jayce or some other ad midlane, you would take the armour seals. and if you didn't, then you deserved the 4 reports your teammates give you for feeding.
it's one of the biggest examples of "we don't play the game we just balance around statistics" which is one of the biggest jokes surrounding the balance team and why they consistently fail time after time again.