We're doing some reporting on student progress in modules, and the data we see in context_module_progressions doesn't entirely match our expectations.

• Sometimes a student enrolled in the course doesn't have a record in context_module_progressions.  What determines when they get a record?
• Some progress records have workflow_state='unlocked' rather than 'started' even though requirements_met shows some of the module items are complete.  Why would that happen?
• Some progress records have workflow_state='completed' rather than 'started' even though requirements_met doesn't include some of the module items.  Why would that happen?