Debuffs to Deflection do not stack with status effects, but debuffs to Reflex do???

The enemy in my test was a Flesh Construct in Sanitarium in Brackenbury. I'm playing at the Path of the Damned difficulty.

Test about Deflection

The Flesh Construct's original Deflection was 54.

After I debuffed it with Aspirant's Mark (which applies -8 Deflection ... etc), its Deflection became 46.

After I further debuffed it with Curse of Blackened Sight (which applies Blinded, which applies -20 Deflection ... etc), its Deflection became 34.

Test about Reflex

The Flesh Construct's original Reflex was 45.

After I debuffed it with Aspirant's Mark (which applies -8 Reflex ... etc), its Reflex became 37.

After I further debuffed it with Curse of Blackened Sight (which applies Blinded, which applies -20 Reflex ... etc), its Reflex became 17.

Conclusion and question

Apparently, debuffs to Deflection do not stack with the Blinded status effect, but debuffs to Reflex do.

Why is there such a discrepancy on the stacking of debuffs?

Edited 11 minutes ago11 min by IanTheWizard