A common problem with gas ranges, mine included, is that they can be incapable of a mere simmer. I have to use a heat diffuser on the smallest "simmer burner" to keep it down to a simmer. Otherwise, it's a steady low boil.
And whether you maintain a simmer or not, may depend on whether the pot is covered or not. No recipe can reliably tell you to simmer on medium.
"Kitchen duty is awarded only to those of manifest excellence..." - The Master, Dogen