Primavera P6

In the P6 Help discussion of "Percents" definitions I cannot find a definition of "Schedule Percent Complete."  As a column heading, Schedule Percent will provide me with the overall schedule percent complete and individual activity percent complete.  I am puzzled, however, on finding activities that sometime show the "Activity Percent" complete higher than the "Schedule Percent" complete, sometimes lower, and sometimes equal.  Why does this happen?  The individual activities may all be defined as "Physical Percent Complete" types when this occurs, if that is a factor.  The "Help" goes into great detail on various types of percents - Activity, Physical, Duration, and Units - but I cannot find any help on a Schedule Percent and the puzzling differences between Schedule and Activity percent complete.