Music is a funny thing, an imperfect thing mixing art and science with lore and legend. For example, if you're playing a song in C and shouting out the chords as they come up, would there be a difference between guitar players who usually play in sharp keys G D A E, and horn player who usually play in flat keys F Bb Eb Ab in the sequence in C,
C Am F if the next chord is a diminished one semitone up from the F, is it F# or Gb?
I'd say F# because it's moving up. In other keys, it might be dictated by whether the key has flats or sharps.
To the original question, that's a good find. I couldn't think of the immediate answer, but as soon as someone said it, I recalled seeing it. Being right next to the key signature without the previous measure, it indeed made no sense.