Hello ! Its says the files included are the :

Kenney Studio 0.4 (Windows)

Kenney Studio 0.4 (Mac)

Kenney Studio 0.4 (Linux Debian)

Kenney Studio (development package) 1.0

What exactly is the development package ?

It's a package that includes the code plus a sample to create your own template files for Kenney Studio, you can read more about that here; https://itch.io/t/16718/how-to-create-template-files-wip

Thanks !