If you're talking about what Arsenal achieved last season, as opposed to what they could achieve this. Well, not very long ago Liverpool F.C. finished the 2017-18 season on 75 points. The following season, when they pushed us so very, very hard, they finished on 97. That is a monumental swing (which no-one saw coming, I think). That Liverpool team was a somewhat better one than this current Arsenal one, in my opinion, but we are not playing very well, really, for the minute. Will we kick into top gear? Will Arsenal start to fold as the season progresses? Who knows?
But a 21-point swing is at least possible. Since a 22-point swing has happened in recent memory.