Is the defiance penalty (reduced XP/Crystals gain) the same for a player who has left a game and for a player who has let the timer expire? The reason why the game ends shows as 'Forfeit' in the first case and 'Time out' in the second case.

The penalty is the same.

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Collectionneur de cartes

Then what will explain that so many high level players wait till the timer ends? If the penalty is the same don't they know that they are just losing time, why do not they just leave directly?
(It normally happens on turn 1 when they see that they are facing The Black Witch with a deck that is helpless against her)

Or is there anything else, for example that the game doesn't give a loss to their statistics if they wait for the timer to expire?

Because somewhere long ago someone said you lose less, and they all believe that comment to still be true. Either that or they are complete dicks, or it could be that one of you guys has a bad connection, or both of you do....but yeah. lots and lots of em...I do it if other person waits till last minute I'll go make a sandwich and let time run out.

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Blarg!!