MHT=mean holding time=(time the SDCCH is busy in the hour)/(Successful SDCCH seizures).
Assignment is when the BTS allocate the SDCCH and seizure is when the BTS receives the SABM message which the phone sends when it goes to the SDCCH channel.
If the BTS does not receive the message it holds certain time before releasing the channel. And durring this time SDCCH is counted as busy, which means traffic (time the SDCCH is busy in the hour).
You shoud have a separate counter for MOC sms/calls, MTC calls, MTC SMS, and location update.
Do you have trains crossing the LU there?