The same probably applies to the shuttlecraft, explaining why our TOS heroes use similar grammar when discussing those: the option in, say, "Immunity Syndrome" is to refit "
the shuttlecraft", singular, for a special mission, even though the ship has at least two craft available. Any one of the generic shuttles may be chosen, but the assumption is that there is always just one immediately available so there is no ambiguity as to which one the heroes should refit, or use, or meet next to, or whatever.
Timo Saloniemi