Definition of Maximum?

I'd like to preposition this post by saying I am a fan of 7-zip. I couldn't help but notice in their Compression level options they have an option above Maximum.

Is there a definition of maximum I was unaware of?

max·i·mum/ˈmaksəməm/
Noun: The greatest or highest amount possible or attained.
Adjective: As great, high, or intense as possible or permitted.

Photo courtesy of my Print Screen button.

I suppose, because it's open source, I can fork it and make my own version that changes the position of Maximum and Ultra. Maybe just remove Ultra all together. There is a decent chance my programming skill is not good enough to do even that (though usually I can find and delete pretty good). I find this over site a little ridiculous for a whole community of logical thinkers, so I can only assume I am missing a key tidbit of relevant information.