Looks like a mistake because of move order. 1. d4 d5 2. c4 dxc4 3. Nf3 is the main line of the QGA I believe.
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I think the problem is that it goes by position. Note that after dxc4, the link for d4 lists 7,131 games. But then after d4, the first choice (Nf6) has 8,013 games. So you're getting transpositions from other openings, I believe Queen's Gambit Accepted. But in the other openings, the move order is different, so en passant is not legal.
edit: whoops, beat to the punch by davegski
[Note: This isn't my topic - somehow the previous owner of this topic deleted his post, and my post got moved up.
I don't even remember what the subject of this topic was. Something to do with transpositions and percentage changes.]