Isn't the problem in calling the amount in your first envelope x. Imagine that the quantities are 1 and 2 pounds in the envelope. In one scenario, x = 1 and by swapping you end up with 2 pounds, but in the other scenario x = 2 and by swapping you end up with one pound, x is variable and not a fixed value.

What is described here would be the situation if you had to decide between a guaranteed one pound OR picking one of two envelopes which contain either 2 pounds or 50 pence (either doubling or halving the one pound). In that situation if you looked at expected winnings you would go for the second option.

Isn't the problem in calling the amount in your first envelope x. Imagine that the quantities are 1 and 2 pounds in the envelope. In one scenario, x = 1 and by swapping you end up with 2 pounds, but in the other scenario x = 2 and by swapping you end up with one pound, x is variable and not a fixed value.

What is described here would be the situation if you had to decide between a guaranteed one pound OR picking one of two envelopes which contain either 2 pounds or 50 pence (either doubling or halving the one pound). In that situation if you looked at expected winnings you would go for the second option.