During the last days I lost a few games by abandonment for no apparent reason. What is the difference between that and a disconnection?
If you get disconnected and don't connect in time, you will lose by abandonment assuming you don't run out of time in the game first.
But there are other reasons a game can be declared abandoned as well.
https://support.chess.com/article/338-how-does-game-abandonment-work
But why does it happen if it doesn't tell me that I disconnected?
Well, the pandemic and more people at home and using the internet increased around that time. The site saw ten years of expected growth in a few months, but it isn't just the site, all traffic internet wide has increased. So, while your local connection may seem fine, the increased traffic in general can cause additional lag, that is being caused by equipment between you and the site servers.
I haven't really noticed much additional load, but depending on the route your traffic takes, you might. The site has been working on improving things, due to the increased traffic, and continues to do so, but can't impact things between you and the site servers.
During the last days I lost a few games by abandonment for no apparent reason. What is the difference between that and a disconnection?