I think you made the same mistake that I made when I first read it. The question isn't "what contains more wine?" but "compare the amount of water in A to the amount of wine in B."

Step 1: A with 10 L wine; B with 10 L water

Step 2: A with 9 L wine; B with 11 L liquid (10 L water and 1 L wine => 10/11 water, 1/11 wine)

Step 3: A with 10/11 * 1/11 = 10/11^2 L water; B with 1/11 * 10/11 = 10/11^2 L wine.

10 / 11*2 water in A ~ 10 / 11^2 wine in B.