LP21 Replenishment

LP21 Replenishment. We have configured a material for replenishment and ran the transaction LP21. We have found that if the material
has a value below the minimum value the replenishment value is incorrect however if the material stock on hand is zero the calculation is correct.

For example;

Max       = 100

Min        = 10

Replenishment =40

If Available stock = 5 replenishment after running LP21 gives us 40 where it should be 80

If Available stock = 0 replenishment after running LP21 gives us correct figure of 80.

Any ideas on why it is incorrect for values > 0 and < Min value?