Hello peeps, specifically mods/admin/ex-mods/eels, I guess

I just wondered why some threads allow you to delete your posts and yet others do not?

I was editing a typo and adding a smilie face to a post and noticed that there was a tick-box allowing you to delete your post, which I hadn't noticed before, so I looked in another couple of threads and you don't seem to be able to do it there.

I just wondered who or what decides whether a post was delete-able or not? Is it the type of thread, age of post, or if you have been quoted or it is the last post in the thread, etc?

No reason, I just wondered if there was a formula to it or is it a bit random (or if the forum was playing up, perhaps.)