Fury works in the combat To the Death, that alone. It can be produced by the table. I can be created by Grenades and Luck. You do not create Fury for say Skirmish. Your weapon has a modifier also in To the Death.
Luck may be added anywhere on any table. It can make the game ponderous, should I use Luck? But in playtest and intent, we allowed Luck to add to any roll even after seeing the result. There was plenty of what-if I add luck, read and then decide.
I'm trying to avoid generalizing (and being wrong). If there is an exception it would be labeled (typically) under a table as a NOTE. Grenades have a counterpart in Gear and Dirty Tricks (other OSR titles). Those Aces do get added in various tables.
Here in USMC at Waterloo, I think we have just the one use of Grenades to modify a roll in To the Death and then keep that modifier for the rest of the battle as Fury.
Now, why are things listed # (or less) and #+...design choice. I understand the confusion. How do you get 6+ here? How does anyone generate less than one on this oddly listed table? Answer is often you won't. But this is an OSR title and nothing is certain with a Referee (GM). I also have found that more gamers have asked me if a roll with a bonus or penalty still uses a number event. e.g., I rolled a three with plus five to my roll, is that a six here? Most are being pedantic.