It has to do with stacking, to yield the greatest debuff benefit, but cast order matters, or we run into a conflicts/block. There's a note in the writeup also to cast Fixation of Ro FIRST, then Blessing of Ro.
Quick Rundown:
Fixation of Ro + Blessing of Ro = 226 ATK, 46 AC
Grasp of Ro = 181 ATK, 42 AC
It's not this cut and dry though. Your DoT lines have debuff components also. This is where some AC values get overwritten. Highest debuff, sharing the same slots trumps lower values. That's why the DoTs were included in the writeup.
Spell Details:
Fixation of Ro:
Slot 4: Decrease ATK by 30 (L1) to 75 (L90)
Slot 6: Decrease AC by 9 (L1) to 22 (L90)
Blessing of Ro:
Slot 7: Decrease Fire Resist by 73
Slot 10: Decrease ATK by 151
Slot 12: Decrease AC by 24
Grasp of Ro Rk. II:
Slot 4: Decrease ATK by 75 (L86) to 77 (L90)
Slot 6: Decrease AC by 26
Slot 7: Decrease Fire Resist by 75
Slot 10: Decrease ATK by 104
Slot 12: Decrease AC by 16