# Why does this line of code give a flat distribution?

I'm struggling to understand a line of FORTRAN code and was hoping someone on here would be able to help me out. These lines of code are meant to generate a flat distribution - I understand that GRNDM(RDUMMY,1) is using FORTRAN's random number generator creating a vector called RDUMMY with an array with length of 1, so that only one random number is generated giving a flat distribution. However I don't understand the function of the second line RND = (RDUMMY-0.5)*SIGMA+XMEAN, why would this be needed to give a flat distribution?

You are omitting to mention what GRNDM actually does (and that turns out to be not so obvious to find on the internet): It returns random numbers in the interval $$[0,1]$$, equally distributed. The formula in the second line then produces random numbers in the interval $$[\mu-\frac\sigma 2,\mu+\frac\sigma 2]$$.
Neither of these are of course "flat" distributions in the sense of having the same probability over the entire range of numbers. Rather, they are flat only in a certain interval (either $$[0,1]$$ or $$[\mu-\frac\sigma 2,\mu+\frac\sigma 2]$$) of the real number line.