I'm apparently getting unlimited Photo Stream storage

So I just opened iPhoto for OS X with an empty library. It asked me if I wanted to use iCloud. I said yes.

Then my Photo Stream starts being downloaded. I take a look at it. I expected to see only a couple tens of photos like I do when I look at Photo Stream in Photos.app on my iPhone or iPad.

Nope. It actually starts downloading hundreds of photos. The oldest ones date from 2009. Some of those photos have never been on any of my iOS devices nor my current Mac, and only have been synced to iCloud on my previous Mac years ago.

What's up with that? Aren't photos supposed to be stored on Photo Stream for 30 days only? Is that a glitch or is there a part of Photo Stream I don't understand?