Mostly it’s a lot of notes taken over a long time, then going through the older games which were mostly physical copies (and thus still in my storage). Also I think I have better memory than most people, at least for things I care about.

There’s a lot of games I’ve played and forgotten, although those were often games I tried and gave up on.

Also the played date on many games is just first day of the year as I later remembered “oh yeah, I played this game around that time” and put it on the CSV. Often me playing a game comes with some other memory: “Yeah we played this with that friend when he was back from the military” and so on, which allows me to binary search the years.