Having given a zillion interviews, I expect that they weren't looking for the One True Answer, but were interested in seeing if you discussed plausible reasons in an informed way, as well as seeing what areas you focused on (e.g., do you discuss compiler issues or architecture issues). Saying "I dunno" is bad, especially after hints like "what about ..." and spouting complete nonsense is also bad.

(I'm just commenting on interviews in general, and this is in no way a criticism of your response.)

I think I said something about the stack efficiency. I was a kid that barely understood out of order execution. Register renaming and the rest was well beyond me. It was also a long time ago, so recollections are fuzzy. But, I do recall is they didn't prompt anything. I suspect the only reason I got the interview is I had done some SSE programming (AVX didn't exist yet, and to give timing context AltiVec was discussed), and they figured if I was curious enough to do that I might not be garbage.

Edit: Jogging my memory I believe they were explicit at the end of the interview they were looking for a Masters candidate. They did say I was on a good path IIRC. It wasn't a bad interview, but I was very clearly not what they were looking for.