Typically, there's a different focus with regards to evaluation between the first and final rounds.
The first round, referring to interviews with manager level, prioritize assesment of technical skills. Hence, more portion of the interview duration is dedicated for case interview.
The final round, referring to interview with partner level, emphasize more on fit evaluation (cultural, personality, work ethics, etc). PEI questions would often take precendence over case discussions on the sessions. Partners aim to see if the prospective candidate's profile is aligned with the future direction of the firm and if they would be comfortable putting the candidate in front of their clients.
Generally, if there is any significant concern over the technical skillset and performance displayed on the first round, the candidate will either be selected out of the pipeline naturally, or if their performance is within the borderline, the partners will try to test it further during the final round.
Consistency is the key.
In summary, the offer decision weights more heavily among the partners on the final round.
If a candidate manage to pass the first round, that means they can safely say they have checked the box on these phases, even with a borderline performance.
What will determine offer extension is whether they manage to perform well during the final rounds, when the partners try to probe further on the borderline performance concerns flagged by the managers as well as excelling on the PEI sections.
Hope it helps.