Quote from Roy's reply:
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Session Length used to take the time of the last hand and subtract the time of the first hand. I've changed it to add an extra 2 minutes (to account for time the last hand might have taken to play).

Minutes Played is different in its calculation. For example, take a session where you play for 10 minutes, take a 10 minute break and then play 10 more minutes. Minutes Played will be around 20 but Session Length will be 30
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