Some companies have to enforce retention policies for business and/or legal reasons, which means they actually have to delete your data if they say they will.

Some sites only “soft” delete things because it’s simply easier and cheaper.

Regardless, I can’t reiterate what you said enough:

It’s why they say everything is permanent on the internet.

Nobody should ever once in their life assume that data they post online will be discarded, ever. Maybe it will, but never assume it will. Even if you run the server yourself and delete the data files on your server and send the hard disks into the sun, if the data was ever accessed, you should treat it as if it’s been captured and retained somewhere.