The quality of play can be impacted by a number of factors. At one tourney he could be at his best, playing awesome chess. At another, not doing so well.
Is that chart based on opponent's pre- or post-event ratings?
The reason that I ask is that some of those low rated players may be improving and may be a lot stronger than their rating would suggest. Also, it is possible that some of the higher rated players might be at their rating floor and playing lower than their rating suggests.
There are other things that might impact things. Higher rated players may play more principled chess and he gets into better or more familiar positions with them and some of the lower rated players leave book early and he may have a harder time with that.
Really, it would take a higher rated player to look over his games (or some really good analysis of his own games) to see if there might be something fundamental missing from his game.
I had a really good tourney last Spring and shot up almost 100 points. The next couple of tourneys I dropped most of that gain. That is the way it goes sometimes.
My brother in law likes to play in chess tournaments, and really hasn't changed in skill level for a few years now. However, looking at his tournament history, I notice that his performance varies by a LOT. in 2015 he went from 1702 to 1914, and then back down to 1729. During that span he defeated 13 players in the 1900-2100 range, and also lost to 11 players in the 1300-1600 range. What could be a reason for this, and what would you reccomend to remedy this? If he only stops losing to low rated players, his rating would jump quite a bit. Here's the table of his stats so far: