Thornmail is giving critical strike damage.

Unsure if this is the case but the only thing I can think of.

Tank Garen top (me) plus Sejuani jungle. We both had a highest crit of 11/62, respectfully. Neither of us have critical strike items. So it should be 0 Largest Critical Strike + 0% Critical Strike, yet that is not the case.

The only plausible though is that when we were crit by Vayne, we returned crit damage back to her.

LCS Damage Image.

Items: Garen - Thornmail, Righteous Glory, Gargoyle Stoneplate, Ninja Tabis, Warmogs Armor, The Black Cleaver Sejuani - Cinderhulk (jungle item), Randuin's Omen, Thornamil, Ninja Tabi, Warmogs Armor, Giant's Belt.

Vayne - Mercury's Treads, IE, Mercurial Scimitar, Rapid Firecannon, Statikk Shiv, Lord Dominik's Regards.