One other possibility is that the average rating of players in online chess websites can also fluctuate quite a bit by the population of the online community.
A good example of this is that I've been playing pretty consistently on chess.com for the last 8 years, and my rating there peaked in the high 1500's, and I seem to be down to the mid 1400's.
I also noticed that this rating fluctuated in a lot of my friends who also play with me on that same community. Over the last 8 years, chess.com's ranked player count doubled, and thus all the ratings of players also shifted as more competitor's entered the community.
Rating points are always going to be relative and fluctuate with the community population as more beginners come into the game, they play and lose against each other keeping their group within the same range, while the players above them can pushed further up in the rating pools.
This is all based on my personal experience having played online for quite a while, and reading over the stats available to me.