Why? At equal level, sure, but when so far above it'd be surprising not to win.

I think the context here is it's an average win rate across multiple games (i.e. playing both white/black an equal number of times per player).

A draw counts as "half beaten."

So 8 wins and 2 draws is winning 9/10.

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A 660 point difference between Stockfish and Carlsen should be a 1 - 10^(-662/400) = 98% win rate for Stockfish.

More than likely, that's 96% win, 4% draw, 0% loss.