I'm going to describe this from memory, so it may not be 100% accurate, but here is how I remember it:

I setup a rubric with three levels per criterian area, that added up to a total of 30 points. For most levels of each criterian area, the scale went 3-2-1. However, there were some that went 3-2-0. Everything added up to a total of 30 points, so I assumed everything was good.

I went ahead and graded 140 projects, told the kids to take a look at the rubrics, and they pointed out to me that their final scores didn't match the point value they got when doing the math calculations by hand.

I was scratching my head because I couldn't figure out why Moodle wasn't doing the math correctly, so I sat down with another teacher and took a look at it.... we determined it was because we didn't have a category for "0". I went back in and added a blank category of "0" to the rubric and manually regraded one student... that fixed it...

I also remember the paragraph above the rubric read:

The minimum possible score for this rubric is **7 points** and it will be converted to the minimum grade available in this module (which is zero unless the scale is used). The maximum score **30 points** will be converted to the maximum grade.

Intermediate scores will be converted respectively and rounded to the nearest available grade.

If a scale is used instead of a grade, the score will be converted to the scale elements as if they were consecutive integers.

I guess the question that I'd like an answer to is why doesn't Moodle do a simple calculation of the total points a student earned (based on the blocks I click) out of the total possible points?