When I've seen this with people interviewing in Amazon, it's generally because they're tagged to a higher level position, and during the interview, they decide they're not a fit at that level, but possibly at the one below it. i.e., he might have originally been interviewing for an SDEII position, but they felt his skill-set was closer to what they look for in an SDEI, but still wanted to make him an offer to join the team.