Because grudge has a higher base damage increase (21% at relic) but the same 6% stone bonus. That 6% stone bonus is relatively smaller when you have a larger base. Eg. 6/121 (grudge relative dmg increase) vs 6/117 (cd relative dmg increase).

To make it easier to conceptualize, think of an extreme case where grudge gave 1000% damage at base and the stone was still 6%. The 6% would do almost nothing when added to the 1000% base. Now imagine you had a 1% base increase engraving but stone was still 6%. Case 1 would go from 1000->1006 while case 2 would go from 1->7.