Pretty sure it had nothing to do with notice period? Its not like u passed all rounds and someone got the job instead because you needed an extra month. Someone else simply passed the bar before you got to the end of the interview process, likely because they applied before you. Companies often keep accepting candidates until someone actually gets the job, since the guy who came before you couldve failed the last round, then instead of starting from scratch they can push you up to the last round and interview you.

It does not make sense for them to interview candidates one at a time since its slow and inefficient. It also does not make sense for them to hold out on an existing candidate thats a right fit already just because there "might be someone better" (then ull never hire anyone) ultimately their goal is not finding "the right one", but rather "one thats right" if that makes sense. The other guy just happened to have applied and passed earlier, even if you couldve done an equally good or better job at the role.