Scripts return result in Min/Max Format. The minimum is initially set to zero and the maximum to 999999. After all the scripts are called, the maximum is used as the Hit Bonus.
The final result will be limited to the target's current hitpoints + 1.
As an example of use: You might define a special ability named 'IsSleeping'. The GetItemHitBonus script for 'IsSleeping' will probably return a large number to ensure the target's death.