How does Twitter decide which replies to show under a tweet?

Say I am following @person_A.

I (let’s call me @me) reply to one of their tweets (starting the message with @person_A).

I can now see my reply threaded beneath their original tweet, along with other replies to them from people who I don’t follow.

However, when I look at the same feed using a different account (let’s call it @pseudonym), which is following @person_A but not following @me, the reply I made using the @me account is not visible, even though replies from other people who @pseudonym is not following are visible.

Why is this? Why are these other people that I am not following deemed to be important enough to display, but not @me? Is there some kind of hidden ranking system at play here, or does Twitter just display things at random?

I did wonder whether it was only showing replies that @person_A had also responded to, but that’s not the case: some people I don’t follow show up even when they haven’t been responded to.
NB I’m using the iOS Twitter app, if that makes a difference.

No clues?

On my normal feed (using Seesmic, Hootsuite) I just see conversations between people that I follow, but it is possible to see more than that with just a string search on @username. Perhaps the app you are using is a bit more flexible about what it allows in your feed?

/wag