OK, I'm thinking like someone who's trying to get software for free. I still don't get it.

If they were modifying an existing valid key, it doesn't seem like people would keep using the same one. Unless people are downloading a keygen or something.

Any more hints?

To me it seems like these keys were shared on a site somewhere and when people Googled for free blurity key or whatever these came up.

If that were true the author would have been able to find the keys, but he said Google failed him.

Maybe they didn't search on Google then, but another forum or site that isn't indexed by Google.

It's possible the serial was posted on a dark net, such as Tor.

Maybe the invalid key was posted on a warez site and everybody was trying to use it.

Besides a keygen, software might be bundled with a list of keys to use when downloading from certain sites.