"Walk me through your final-year project" — what are interviewers really checking?

This question has come up in every technical round I've had, and I think I'm answering it badly. I usually go chronologically: we chose this topic, then we did the literature survey, then we built X, then Y. By the time I get to the interesting part, the interviewer is already asking a follow-up.

From reading around, the better structure seems to be:

1. One line on the problem and who it's for.
2. What you specifically built or decided (not "we").
3. One real technical difficulty and how you solved it.
4. The result, with a number if you have one.
5. What you'd do differently.

The follow-ups I've actually been asked: why this database, how you tested it, what happens if the load doubles, and what your teammates did. The last one threw me.

What follow-ups have others been asked about their projects? And how deep do interviewers expect you to go on a team project where you only built one part?