The f object and f class The f object is the unique instance of the f class; the two are distinct objects. The latter is also a parsing word which adds the f object to the parse tree at parse time. To refer to the class itself you must use POSTPONE: or \ to prevent the parsing word from executing.
On the other hand, t is just a word, and there is no class which it is a unique instance of.
t \ t eq? . t
Many words which search collections confuse the case of no element being present with an element being found equal to f. If this distinction is important, there is usually an alternative word which can be used; for example, compare at with at*.