inacuraccy during the openning even when playing book moves

So I played King's Gambit, Falkbeer Counter Gambit in this game (that's what I usually play vs. King's Gambit) and I noticed that 4. ... e4 is considered by the engine as an inaccuracy, although it's a book move. Mainline, even.

Maybe you should consider analysis' settings to favor book moves.

It's not because it's a "book move" that it's a good move.

Engines are weak in openings without their opening book. You can ignore engine comments as long as there are games in the masters' database.
4 d4 is an inaccuracy and not labelled as such.

evans gambit also contains an inaccuracy in the opening

Well, the Evan's Gambit is unsound...

A position can only be a win or a draw. So what is an inaccuracy? Does it spoil the win? ;)

Btw. 1.d4 g6 is one inacc. as well.