Why are sensors sometimes combined as an open/closed event and sometimes separate in the activity history?

A sensor's Open and Close events typically get combined into one line in the activity history when the events occur within a few minutes of each other.  

If a sensor is opened and then not closed again for a longer period of time, there will be two separate events in the activity history: one for the sensor being opened and another for the sensor being closed. 

For more information about configuring Sensor Activity Monitoring for sensors, see Turn sensor activity monitoring on/off.