Different values using a masterwork/enchanting ring ...

Wearing a ring should raise the chance of masterwork/enchanting.
The rings are telling +5, but changes on % are different, while using rings:

Alchemy Enchantment (Ring of Chrysopoeia):
1. try: +95% --> WITH ring no change: still 95%
2. try: +76% --> WITH ring +77%
3. try: +40% --> WITH ring +41%

Tailoring Masterwork (Thimble of Evelyn ring):
1. try: +95% --> WITH ring no change: still 95%
2. try: +76% --> WITH ring +77%
3. try: +40% --> WITH ring +41%


What's the meaning of '+5', if it grants 1% on second and third try, but nothing on first try?
It must be at least +1.25% to get +77% / +41%.
But if so, it should be +96% at first try.

This should be a bug I suppose?