Care to explain why I had so much Magic Resist?

In a game yesterday where the enemy Riven got fed, so I stacked armor and bought almost no magic resist (I was jungle Hecarim). The game went late (we won at 1:14) and obviously eventually the enemy Fizz was full build and since no one on the team built MR he was doing a lot of damage. You know, except to me. My teammate had said "Thresh you need to build MR" and I thought "oh I probably need to too, I only have Mercury Treads" and then looked at my MR and it was 297. That seems awfully high, doesn't it?

Anyways, I typed in chat that I had 297 MR for some reason and no one said anything of it - so I kind of thought it must have been the right amount. I don't play tanks often, I figured maybe Hecarim just gets a lot of base MR by level 18.

So I looked it up. At level 18, Hecarim has under 60 base MR. I had 9 scaling MR glyphs, so that's an extra 27.

My build was (MR): Stalker's Blade - Warrior (0) Iceborn Gauntlet (0) Randuin's Omen (0) Mercury's Treads (25) Sunfire Cape (0) Frozen Mallet (0)

I had also drank about 7 elixir's of Iron throughout, so maybe one of those was active (also 0 MR though).

So yea, I had over 400 armor as intended. I also had like 1650 bonus Health (which was my intended defence against magic). Just wondering how I had nearly 300 MR when my build and runes add up to roughly 110? Masteries should have added no more than 20 as well.

My masteries were 0/21/9. I did take the 5% bonus armor and MR mastery, as well as 3 points in Legendary Guardian. I did NOT take the 3 points for +5 MR (I took the +5 armor instead).

My teammates were: Ezreal, Lux, Nidalee, Thresh

As far as I know (and I just looked them all up as well) none of them have an ability that boosts MR. I also looked at the detailed match history and none of them had any items that boost their allies MR.

List of items they had: Trinity Force, Zephyr, Iceborn Gauntlet, Infinite Edge, Guardian Angel, Statik Shiv Void Staff, Rabadons Deathcap, Zhonya's Hourglass, Sorcerer's Boots, Rylai's Crystal Scepter, Morellonomicon Zhonya's Hourglass, Randuin's Omen, Rylai's Crystal Scepter, Rabadon's Deathcap, Sorcerer's Shoes, Trinity Force Boots of Mobility, Randuin's Omen, Righteous Glory, Sunfire Cape, Frozen Heart, Thornmail

Just wondering why I had 297 MR (even if it was only for the moment I glanced at it - although I think it was permanent). I also had over 400 armor (so I didn't accidentally look at armor, it was definitely MR that I was looking at). Is there something that turned bonus health or armor in to MR? Is there some bonus from smiting something? Am I missing something obvious? Tier 2 turrets were destroyed so I don't think they contributed.