In the D&D 3.5 Monster Manual, a Hill Giant's damage entry shows:
Greatclub +16 melee (2d8+10)
A Hill Giant's strength bonus is +7. Since there doesn't appear to be any magic abilities or feats that increase the bonus to damage, where does the additional +3 damage come from?
The Frost Giant has the same problem. Their damage entry shows:
Greataxe +18/+13 melee (3d6+13/×3)
However, their strength modifier is only +9.