"All my updates" is a combination of "My favourites" and "My content".
"My favourites" is any updates to questions you added as favourites.
"My content" is any replies to your posts. Replies are:
- comments on your question
- answers on your question
- comments on your answer
- any comments following a comment you made
So if DT posts an answer and trachy comments on the answer, then Speed freak comments after that, both DT and trachy would see that question at the top of their updates.