Response times are recorded for a specific LU rather than for a specific 3270 session. This means that if a 3270 session ends and another session (of the same or a different 3270 user) then uses the same LU, the response time counters will include responses from the previous session as well as those from the new session. This does not happen, however, if the RTM counters were reset before the new session began to use the LU (the counters are reset when the host requests that they be reset or when RTM data is sent unsolicited by SNA Server). When hosts request RTM data they typically also request that the counters be reset.
If you want to ensure that the RTM data displayed relates only to the current 3270 session, you should select the RTM Data Sent at End-of-Session checkbox when configuring RTM parameters for SNA Server. This will ensure that the RTM counters are reset at the end of each 3270 session and will therefore be 0 at the start of each 3270 session. The host can, of course, override this setting.