There are three legitimate reasons:
The Arsenal team was a no-show — Rules indicate that if a team fails to show up for the game, the team present is automatically granted a 3–0 win.
The Arsenal team lost enough of its players, 5, for them to be forced to forgfeit — Rules again indicate that if a team loses more than 5 players, or has less than 6 players playing on the field, it is forced to forfeit the game, regardless of the score, and the other team wins by 3–0.
It was a second leg of a two-leg tie — Chelsea already hit 3 past Arsenal in the first leg, and drew 0–0 in the second leg.
And the final reason would be that it was a game between the two clubs’ women’s sides, or was a U15 game, in which the players cannot be really considered “man” or “woman” in terms of age. They are theoretically still children, playing football for a professional football club.