I’m not sure exactly how the Fediverse Observer calculates the day-to-day totals, but my speculation for the dips in the graph is:
Some of Lemmy’s servers/instances going offline could cause the total to go down;
Post deletion could also contribute to the total going down.
These dips in the graph confused people in my previous post, as the dips were suggesting that the graph represents ‘new posts by day.’ I edited the title in the screenshot this time to clarify it is total by day.