Explain why they would split it into 2 categories...

When using Disciple of the Hand equipment and the appropriate abilities, Quality will improve during synthesis by 300 to 600 points.

(Example)
When all items are HQ items
Final Quality will be between 800–1,000 (odds of producing an HQ item range from 50% to 100%).

When all items are normal items
Final Quality will be between 300–600 (odds of producing an HQ item range from 0% to 10%).

* Actual figures are still under development. They may be adjusted or changed depending on feedback.
Wouldn't this best be written as...

Based on ending quality...
600 quality = 10% chance of HQ
800 quality = 50% chance of HQ
1000 quality = 100% chance of HQ

If you use all HQ ingredients, you start with 500 quality.