There are challenges for the longest sequence of moves in which white's available moves increase or decrease. This challenge is for the longest sequence of moves (from the opening position) in which white's available moves stays at 20, without repeating any moves* or positions. (The fact that a given move can't be played for the purpose of the challenge doesn't prevent it being scored as an "available move").
For instance, after 1. b4 b5, white still has 20 moves available. The pawn has lost two moves, while the dark-squared bishop has gained two moves.
* "repeating a move" means moving the same exact piece from the same square to the same target square, and whether it is a capture: "the knight that started at b1 moving from c3 to e4, not capturing". Other considerations like check do not stop it being the same move. (The rule is to avoid situations where a number of interchangeable moves are played semi-repetitively without changing the board meaningfully).