Normally Savvy reduces Isabela's damage by 10% per enemy multiplicatively, therefore the penalties are not applied as a flat rate but in sequence after one penalty has already reduced the damage. Effectively, each subsequent penalty is reduced. This prevents Isabela from dealing 0% damage should she be near 10 enemies or more. For example, facing 6 enemies (a reasonable number given the distribution of enemies) Isabela's damage is reduced to roughly 53% of its default level. This upgrade cuts the penalty in half to 5% per enemy, with the second only removing 5% of 95% of her damage, and so on.
Damage per nearby enemy w/Savvy active (as a %): 90, 81.00, 72.90, 65.61, 59.05, 53.14, 47.83, etc.
Damage per enemy w/Bucc's Savvy active (as a %): 95, 90.25, 85.74, 81.45, 77.38, 73.51, 69.84, etc.
Essentially, the damage penalty is lessened in larger groups, and the upgrade refunds an increasing percentage of damage with increasing numbers of nearby enemies, starting with "half" at one enemy.