What is the rhyme and reason for when some posts say "Last edited: blah blah"?

The reason I ask is that in some cases I edit my post for spelling or something and it makes the notation on the post, and other times it does not. I assumed it was for when posts are edited after a reply has been made, but a post the I quoted today was modified after I quoted it, and there was no indication that it had been edited. (The part I quoted was not edited however). So, what is the deal?

You get a certain amount of time, I think it's 10 minutes, after posting to make corrections without the "last edited" note appearing. For another, again I'm not sure but I think it's 20 minutes, you can still edit but the "last edited" note will appear.

The idea is that making quick corrections to your post after seeing it go live doesn't need special notice, but making changes later, possibly after responses have been posted, does need to be noted.

Thanks for the clarification. It was a quick moving thread. I should have guessed it was based on time.