Without going into too much detail they are separate modifiers.
The item with 8% to parry modifies your parry skill from 280 to 307.
The Improved parry III item doesn't actually mod your parry skill, it gives you an added 24% to whatever your base chance to parry is.
Purely HYPOTHETICAL example.
Say your parry skill (MOD'd with the +8% item) of 307 works out in game to give you a base chance to parry 50% of the time that check is performed. (yes I KNOW this is nowhere near the correct amount, I'm just using it as an easy example.)
Improved Parry III would give you an added 24% chance of that base chance.
In this example it would give you a 62% chance to parry if that check is performed.
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Adita Bedsnake
85 Trailwarden
The Gameless server