When I saw the deck size, I thought I would win that one fairly easy.

For some reason people with 100+ card decks never fall victim to probability and variance. By all rights they should end up either mana screwed or mana flooded just about every single game, as a 250 card deck probably has at least 90-100 lands (any less and they're insanely lucky to see more than 2-3 lands a game. So that means that there should be plenty of scenarios where they could end up hitting patches of 10+ lands in a row. And with 150 or so spells, it seems very likely that they'd also see streaks where they never see a single land aside from their opening hand.

Add to that the fact that they should never honestly expect for any kind of consistency or even see relevant spells when they need to cast them. Mana curve? Forget it.

But some people are just naturally lucky.