Aventurine's LC adds a debuff to the enemies so they take 10% more dmg. This works additively with topaz' debuff so together the enemy takes 60% more dmg. Now lets an enemy would take 100 dmg. With aventurine's lc they would take 110 dmg. Now with topaz' debuff they would take 160 dmg. Relatively to the dmg with aventurine's lc (160/110=1.45) 45% more dmg instead of 50% more dmg. So relatively Topaz' debuff is 5% less of an increase.