This is not just a matter of vernacular. Nullary connectives such as ⊤ and ⊥ do not usually behave the same way as atomic variables: for instance, while the latter allows for consequence-preserving (uniform) substitution (check here the notion of a formal logical system over a substitution-invariant consequence relation) the former does not.
Note: if you do not like the terminology "atomic variables", above, you may substitute it by "propositional variables". The whole point of my anwer is the need of distinguishing this class (whatever you call it) of syntactic objects that allow for consequence-preserving substitution from the class of somewhat similar objects called "nullary" / "0-ary" / "0-place" connectives, that do not in general allow for such forms of substitution. In many aspects the difference is analogous to the difference between a variable and a constant.