Sanity Group interview questions

I had the first interview with HR and it went perfectly. I felt relaxed and the interviewer was friendly and warm. I got the feedback at the same day, which is positively rare, and was invited to the second interview. The second interview was with my direct manager. I expected the atmosphere to be as friendly as the first interview. Anyways, it went well too. He gave me a brainteaser to solve. Frankly, I didn't perform perfectly under stress and some calculations slipped off my hand. But when I reached the final answer, he said it's pretty close. However, after almost two weeks I got a rejection. Although it was a great chance to get to know the team and the company and I enjoyed talking to them, I find their evaluation a bit unnecessarily strict.