That's just an artifact of how I made this (I just used python to stat the files and check their mtime, and not their birth time since python doesn't easily allow you to do that) and the fact that I've moved this collection from device to device a couple of times, so some timestamps are not really accurate. So a lot of files that are much older than their mtime show up as last modified on a certain day.

I think most of the files have an mtime of 2013, which is when I bought the first external hard drive this media collection lived on. It's grown over time and moved to different devices; currently it's on a NAS that has about 12 terrabytes of storage that I created in the early part of the pandemic. SInce it's SO much bigger than what I had before I ended up starting to download bigger files so the rate of increase shot up after I moved it to this NAS.