I see two possible questions here.
A1: When students take a single-variable calculus course, they do not think of it as single-variable calculus. It is just calculus to them. Only later they may (or may not) get the appreciation of the multivariable calculus as a somewhat different subject. Depending on whether they do that, they will tag questions as calculus or as multivariable-calculus.
A2: It can be, but this is not necessary. Most multivariable-calculus are not tagged calculus (we have $4509$ questions with multivariable tag only, versus $1211$ with both tags). Given the large number of multivariable-calculus questions, one can be reasonably confident that users interested in following this tag have added it to their favorites, along with (or instead of ) calculus.