I can see why it would throw someone off. There is no Eb in F major (it's supposed to be E natural). Strictly speaking ("diatonically"), the only notes in F major are F G A Bb C D E F. That's it.
In classical music terms, using chords/pitches that are not in a key is called "mode mixture." Happens all the time in black music. Also, whenever you lower the 7th scale degree (in this case, E) so that it's a whole step beneath the 1st scale degree, the resulting pitch is called the "subtonic" (in Western theory).
Taking it back, George Benson goes back and forth between the subtonic and the tonic (b7 and 1) chords in the recording, "On Broadway."
So, short answer, sometimes people still refer to a song as being in a particular key, even though they might vary from it a bit.