Comment by chessroots

You're very likely correct. The Lichess dataset that we have used contains almost all live games played on the site. As fast time controls seem to be more popular on Lichess generally, and take less time to play, we do see a lot more of these in the data. If you'd like to filter the dataset to different time controls or skill levels, you can click the graph button at the top of the page, then click dataset from the drop-down.