Damage and armor modifiers are handled by integers so the decimals don't mean anything. What you're probably missing is the bonuses of More Ranged Damage or Bonus Ranged Damage.
So the base damage is 30-40.
More Ranged Damage gives you 33-43.
2 DT gives you 31-41.
10% DR is applied and rounded down, you're at 27-36.
Fast shot adds 5, leaving you with 32-41.
You gave no information about any relevant perks or weapon mods that would affect the damage, so I'm gonna have to assume you took some ranged damage perks because otherwise something is awry. Either way, this is not a bug - only some confusion about the various hidden oddities in the combat script (understandably so).