This is not the same as setting a variable to nothing, although it is sometimes hard to tell the difference. One way to tell is to use the set built-in with no parameters to list all current variables:

$ FOO=bar
$ set | grep ^FOO
FOO=bar
$ FOO=
$ set | grep ^FOO
FOO=
$ unset FOO
$ set | grep ^FOO

Using set with no parameters like this is similar to using the export built-in, except that set lists all variables instead of just those marked for export.