Why are we unable to enter Physical and Units % complete independently

• Activity Percent Complete: Enter the percentage of work performed on the activity. If the activity's percent complete type is Duration, Units, or Scope, updating this field will also modify the corresponding duration, unit, or scope values. If the type is Physical, the percent complete can only be modified using this field.

My reading of this is that to change the physical % complete of an activity, you HAVE to have the activity's Percent Complete Type set to Physical.

If i do that, i can't amend the Units % Complete

The two values are independent of each other.

If i have done 20 hours of a 40 hour task, it does not automatically mean that i have completed 50% of the task.