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?