"just as"

Reading the Ford-Battles edition of Calvin's Institutes, I have to remind myself that it seems "just as" often seems to mean "kind of like in some way or ways" rather than "just and precisely as in every respect". I imagine one could get into hot water pretty quickly, for example, if one pressed this "just as":

... by true partaking of him [Jesus], his life passes into us and is made ours - just as bread when taken as food imparts vigor to the body. (Institutes 4.17.5, p1365).

Right, enough distracting myslef from typing out the Instiutes. Back to it...