I cannot see your images here.
The way the grades should be working, is as follows:
* Get all the items in the checklist that are not user-specific, not optional and not hidden
* For those items and for each student, check to see if they are in the same grouping
as the student (or if no groupings at all are used, then the item is available for the student)
* For all the items found above, count how many are checked by the student (student-only checklist) or by the teacher (teacher-only or student and teacher checklist)
* Calculate the grade for the student as total number of relevant items checked, multiplied by the maximum grade for the checklist, divided by the total number of relevant items in the list
The code is available here: https://github.com/davosmith/moodle-checklist/blob/master/lib.php#L228
if you want to read through it yourself.
I've not had any reports of problems with this (and the code is mostly 5-10 years old, with one minor variable rename a couple of years ago), so I'd be surprised if there was something fundamentally wrong with it now.
Can I suggest you carefully re-check the data, just in case you've missed something in the configuration?