completion tracker bar block report

I'm hoping you might be able to help me understand something. On course in moodle I've just added a progress completion block so we can track students on specific tasks. However, I can see that for many of the students, the tracker is indicating that they have completed tasks. Some of them are indicated as having completed tasks even though they have never access the module!! Only two of the activities are accessible to students so it should be impossible for them to complete more than the first two blocks.
Can you offer any suggestions? It's important that the tracker is working accurately because it is part of how student attendance and engagement is tracked in this module.

I'm just following up on the issue I raised below. I have one additional observation about this issue, I've checked the Gradebook and there are no results for the tasks which the Completion Progress plug-in has marked green.