What is the unit of `Power_On_Hours` in `smartctl`? Macbook Pro, OS: 10.13.6 (17G65)

9 days ago, my smartctl's output:


ID# ATTRIBUTE_NAME          FLAG     VALUE WORST THRESH TYPE      UPDATED  WHEN_FAILED RAW_VALUE
  1 Raw_Read_Error_Rate     0x000f   100   100   000    Pre-fail  Always       -       0
  5 Reallocated_Sector_Ct   0x000f   100   100   000    Pre-fail  Always       -       0
  9 Power_On_Hours          0x0032   100   100   000    Old_age   Always       -       1638


9 days later, today's output:


ID# ATTRIBUTE_NAME          FLAG     VALUE WORST THRESH TYPE      UPDATED  WHEN_FAILED RAW_VALUE
  1 Raw_Read_Error_Rate     0x000f   100   100   000    Pre-fail  Always       -       0
  5 Reallocated_Sector_Ct   0x000f   100   100   000    Pre-fail  Always       -       0
  9 Power_On_Hours          0x0032   100   100   000    Old_age   Always       -       1644


The macbook has been on from about 9:00 to about 22:00 everyday. The value from 1644 - 1638 = 6, it's definitely not hour, so what is it? 

Edited:

If the smartctl's result in MacOS is incorrect, can anyone suggest me another tool which can check the correct harddrive's power on time? Thanks.
SMART Monitoring Tools

The smartctl tool claims hours, so that suggests one or more of the following:


A fault in the tool;
A fault in your drive reporting firmware;
Or that your drive was only active for six hours during that period.


Drives will power up and down whenever they can to save energy. Six hours of storage access may not be unrealistic but it all depends on how the computer is used.

Try using the -v flag to customise the output. You can specify times in minutes or other units. This may give a better insight into the reported values.

If you need help using and understand the results of smartctl, please contact the developers. They are best placed to provide authoritative answers.