Hi @Richard_Allen ...
I think your analysis of rubric ranges is incorrect. I created a new rubric in my own sandbox course sort of based on values that you detailed in your question:
You'll notice that each of the cells has the ">" symbol. So, ">80 pts" in the left-most cell is not the same as "80" in the next cell to the right. ">60 pts" is not the same as "60" in the cell to the right of it...and so on. So, if a student were to score 80.01 points, that would be "Great", but 79.99 or 80.00 points would be "Good".
More detail can be found in this Guide: How do I add a rubric in a course? - Instructure Community (canvaslms.com)
I hope this helps to answer your question. Let Community members know if you have any other questions about this...thanks!