Repeating... the numbers shown are not the real numbers. At least not for infantry fire against other infantry from what I've seen.
If it says 1% for a tank firing the main gun then yeah 1%. Ditto SPAA. Infantry squad firing all of their infantry weapons at another infantry squad the odds are higher. Exactly what they are we do not know.
Don or Andy could answer this better. In all likelihood a forum search would reveal the answer
EDIT: From MobHack Help
Class 1 - This is the infantry primary weapon class. This covers Rifles, SMG and so forth. Infantry units with a class 1 weapon in weapon slot will have their fire effect adjusted for the number of men in the unit and range in hexes, less bonus at long range. This reflects the number of riflemen in the section. Inf Prime weapons not in slot 1 do not get this bonus.