Meta Research Scientist interview questions

I interview for the position of Research Scientist (Systems and Infrastructure). It included 2 phone screening consisting of coding (2 problems each). The onsite interview consisted of 2 coding sessions (1 problem each, with extensions), 1 system design and 1 behavioral (also known as Jedi). The 2 coding interviews onsite were easy. The problems were significantly easier compared to the screening exercises. I did well and I was told I did well in these interviews. The system design was not so hard: it was about designing a system that counts adds click events, where the scale is 100 billion events per day. Although I think I proposed a system that solved the problem and would be able to scale, I was not so coherent and I felt I should have done a better performance analysis. Indeed, the feedback said that I could have done better in this interview. The Jedi interview was about questions related to my PhD (e.g. how I managed my supervisor, how I managed my colleagues, how I managed conflicts, etc.). The biggest surprise for me was when I received the feedback stating that the interviewer was not able to get technical details about my PhD. I felt this was not fair because I covered all my answers with examples from projects done during my PhD. I do not know how deep the interviewer wanted to go into technical details. This seems quite a strange reason to reject a candidate.