It will depend on the timing of the registration attempts, if they are quick and the SM is still in the current session database at the AP you will get a re-reg. If it is longer it will be classed as a new registration and new session. So it sounds like the SM is dropping out and taking a while to come back in your case.
If you check the advice below (from user guide) and if you are still seeing problems then we could look at getting some engineering files to see why the link dropped.
Re-Reg Count When a SM/BHS makes a Registration Request, the AP/BHM checks its local session database to see whether it was registered earlier. If the AP/BHM concludes that the SM/BHS is currently in session database, then the request increments the value of this field. Typically, a Re-Reg is the case where both: • SM/BHS attempts to reregister for having lost communication with the AP/BHM. • AP/BHM has not yet observed the link to the SM/BHS as being down. It is possible for a small period of time if there is no downlink traffic and AP/BHM still assumes the session is up, but the SM/BHS, loses session and quickly re-connects before the AP/BHM knew the session had dropped. This is how a re-registration happens. If the number of sessions is significantly greater than the number for other SMs or BHS, then this may indicate a link problem (check mounting, alignment, receive power levels) or an interference problem (conduct a spectrum scan).