Yes closed is normal.

I cannot confirm the below but this is the norm for most units.
The defrost cycle time is a fixed time usually about 30 minutes.
The defrost thermostat should be closed when frozen. It opens just above freezing. It is wired in series with the defrost heater.

So lets say a defrost cycle starts the heater heats up as the defrost thermostat is closed. But lets also say that all the ice/frost is melted off the evaporator coils after 15 minutes. The defrost thermostat opens killing power to the heater. This prevents extra unneeded heat from getting into the freezer during defrost. The defrost cycle then continues for another 15 minutes but the heater will be off.

I cannot say why the unit seems to vary the run times.

Check the door and seals.
Looks like the unit has an energy switch. Check that someone in the household is not turning it on/off.
Check/clean the condenser/compressor coils under the unit. Also the vents in the kick plate and rear cover.
Check that the condenser/compressor fan is running at the same speed all the time.
Most of the above should effect the unit all the time.
Intermittant problems are a pain.

You may want to get a couple thermometers so you can monitor the freezer and fresh food compartment temperatures. Could be that the unit is varying the temperatures and/or set points.