Enrolment

I pull my moodle data with crystal reports.  My report looks particularly at the course completions table.  For some reason crystal hates the enrollments table, so I leave it out.  I recently found one particular course was reporting as incomplete for about 100 people who are not enrolled. 

After a lot of head banging, I found those people were enrolled in the course at one time by mistake and were unenrolled.  So I switched my report to look for specifically entries where the timeenrolled in the course_completions table was 0 and replaced the time enrolled with 0 for those people in that class.  Then the course disappeared from my reports...

...and so did 16000+ unrelated entries.  SO I looked again in the table and found a whole lot of people who are enrolled but have a 0. In these courses, Enroled users shows the time they were enrolled (not 0) for the specific enrolment method.  I'm unclear as to why timeenrolled would show as 0 when the time enrolled is clearly in the database...somewhere.

Is it possible these people were enrolled before course completion was turned on or criteria was set?  Would that cause a problem?  Or is it something else?   HELP!