Full Armor + HP bulids still easily effective vs. Magic Damage but weak to AD

http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2533395772/215026110?tab=builds

The only amount of Magic Resist I built in this game was from Locket, and that barely put me over 100 Magic Resist. Otherwise, every items involved Health or Armor and I was sitting around 300 armor and 4100 Health. Thought that would be more than enough to let me tank Caitlyn, Yi, or Yorick, right? Well, sort of wrong. Only Caitlyn was a problem.

I was perfectly fine tanking up Orianna's damage with a measly 100 Magic Resistance and 4k Health, while the same health and 300 armor allowed me to pretty much melt from Caitlyn. Her build involved crit and penetration so I get it, she'll still hurt. But the point of the Tank rework I thought was to make it where if a tank focuses their build for a specific purpose, it will work towards that purpose and be weak to other forms of damage. But apparently no, just building health and armor intended to make me beefy to Caitlyn and weaker to Orianna had to opposite effect, I was beefy to Ori and weak to Caitlyn.

TL;DR Full builds dedicated to lessening damage from a specific target should do just that while leaving me vulnerable to others, but doesn't.