[jeffnc]

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As long as our opponent isn't doing something weird with, like, a pair of aces, there isn't a single hand we beat that he should be raising this river with given his previous action.

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But you are completely discounting the possibility that he's doing something "tricky", or "fun". Or, as Mike Caro has pointed out, there is a certain amount of randomness to ALL players, and ALL players occasionally do something more or less on a whim.