Thought my 1st round interviews went well but got rejected. What did I do wrong?

I had my first round interviews (2) at Oliver Wyman recently and I thought they went pretty well. During the fit portion, the interviewers seemed to like what I was saying (nodding, making small remarks of agreement) and didn't seem put off by anything. During the case portion, I led the case actively, I asked for the right data, calculated all the numbers right and the interviewers seemed to agree with my calculations. Explained the thinking behind my assumptions, gave analysis on all the numbers. I ended with a conclusion, supporting data points, and next steps/risks. At the end I asked questions and I thought we had a nice conversation going. I felt pretty positive coming out of both interviews so I was incredibly disappointed when I got the rejection call. I've emailed to ask for feedback but haven't gotten a response yet. Where did I go wrong? I'm thinking maybe because I did calculations manually on paper (there was a moment I had to redo it because they interviewer thought I was getting a different number when it was actually correct), maybe I didn't go in depth enough/wasn't creative enough/just did the bare minimum. I'm just confused and sad. Would really appreciate any insights.