WrenXIII, it looks like the damage system doesn't actually subtract health, but instead only does addition. Therefore to do damage, and take away an opponents health, you have to add a negative value. So the -1 to -0.1 adds that negative value to the targets current health, taking damage away.
If you changed the max and min to both be positive, I would expect that you'd make a weapon which adds to the targets health for every attack.
For a really strange device you could put max at -1, and min at 1, and have something which damages half the time, and heals the other half!