It's ok, those kind of things bother me too
I definitely recall this being an issue, but I usually use the web site to listen to lessons these days, so I had forgotten about it.
Doesn't iTunes rename podcast files according to the metadata? If so, then the filenames should be consistent unless the metadata is not formatted consistently, as Erich said. I think it's just that the naming scheme has changed over time from season to season.
If that's the case, you can use a program like mp3tag if you're running Windows to batch edit the metadata and then use that to rename the files.