Something similar happened in my first job out of college.

During an interview, the person goes over a problem related to graphs. Then asks me if I could come up with a solution. I told him I had trouble following the details and the interview wasn't long enough for me to both figure that out and solve the problem.

I still got the job. I still don't know why he asked it of me. Eventually I found out that it was an actual problem he was trying to solve for work, and it relied upon a lot of domain specific knowledge that I didn't have at the time.