The description of the skill is another file called Skilldesc.txt
Checking it out I can see someone screwed up something. If you check the description of Dragon Tail, for reference, it is written:
StrSkillRadiusSingular|StrSkill18|par3*2|3
par3 is the radius of Dragon Tail on skills.txt, then the description is multiplying it by 2, then dividing it by the number in the next columnn, 3.
Fists of Fire radius is controlled by par1, but in the description, skilldesc.txt is missing the math, so unless for some reason FoF uses the radius information internally different from every other skill in the game, the tooltip is simply wrong on D2R.
Also, FoF has a “par2” which controls the radius per level of the skill, but it is currently set to 0. Just saying, I wouldn’t mind a “1” in there, and par1 changed to “ln12/2” or “ln12/3” (ln12 means par1+par2*(lvl-1)).