When you go to websites like chess.com and analyze your games, you'll notice that most of the first moves are marked as being "book moves"—which I assume are considered equivalent to each other when it comes to engine evaluation, since they're just different ways you could play the game in its early stage, and you merely pick one without your move necessarily being "wrong" (unless it's a total blunder than doesn't belong to any sound opening, of course). For example, after
1. e4 e5 you could play
2. f4 or
2. Nf3, and the computer will tell you they're both top moves in this position without marking either of them as an inaccuracy or a mistake.
But why doesn't this happen in the middlegame? In pretty much all positions, the engine will always agree that there's one and only one move that is best for you. Yet the middle game is the most complex phase of the game, isn't it? So it's where I'd actually expect lots of moves to be fundamentally equivalent to each other, but instead the contrary happens.