Variable fonts are an evolution of the OpenType font specification that allows a single file to contain all of the variations of width, weight, slant, italics, and virtually any other permutation of a typeface the type designer want’s to expose—all in a single file.
The grid system is an aid, not a guarantee. It permits a number of possible uses and each designer can look for a solution appropriate to his personal style. But one must learn how to use the grid; it is an art that requires practice.