Hi YukaDora,
Performance sessions will show all the sessions that are present on the firewall on that particular card.
This includes all the sessions which may have been opened earlier as well but got refreshed due to cntinuous traffic.
The session creation per second shows only the new session that has been created (in last 96 seconds as shown). This exclusively for the new sessions that have been formed in that period and any session that was created and is still being used at 97th second will not show in this counter. Thoough that session will be part of the output under performance session.
HTH