From my experience, up to X damages is the damage you would do from your weapon base damage plus your abilities if you don't shoot a cover. This should explain de +1 of your example. Note that you can always press F1 to see the soldier profile.
On top of this, you will have to addition the critical hits if any. Obviously, you can do less damage: hitting covers, damage dissemination due to distance (to be confirmed, but I think of shotguns for example), damage negation from the enemy (I know there is a perk like this for your heavies, so maybe aliens may have it too).
I tried to look for game mechanism details, but they don't seem to be available. However, probabilities are summed up in the "More Info" button when you are about to shoot.
As far as a "normal difficulty buff" is concerned, I doubt there is one. But I don't have so much experience in various difficulties. EDIT: there may be a bonus depending on your game difficulty as suggested by the options described for the .ini file at this source.