Crosspost to Stats.SE.
I play online bullet, mostly lichess, sometimes chess.com. Once I noticed that my average performance varies depending on the time of the day: e.g. at the morning I can lose 50 rating after a dozen of games, but I will easily regain it at the evening. One possible explanation is that my brain may work worse at the morning because of sleepyness or something. But can the reason be not me but my opponents?
Imagine two countries, A and B, in different time zones. Players from A and from B tend to play chess at the evening; let it be 00:00 UTC and 12:00 UTC respectively. And, for some reason, players from A are generally stronger than players from B. In the extreme example players from different countries never meet each other, so their ratings establish independently. So even if two players have similar strength, the player from B will have higher rating (because he is stronger among his population).
In this setting, if I play at 00:00 UTC I will meet opponents from A, and if I play at 12:00 UTC I will meet opponents from B. Players of my rating will have different absolute strength in these cases.
Another (imaginary) example about days of the week: it could be possible that on average stronger players tend to play on weekdays and weaker players on weekend, which leads to the similar effect.
Have anyone ever analyzed this kind of statistics and is the phenomenon in question possible, or it is believed that ratings are homogeneous in this sense?