I am most definitely going to try this out on my keyboard to see what you mean. Thank you for your educating...that's exactly what I was hoping for when I started this thread.OK.
It means that you are not playing 9 but playing b9.
More about that question playing Locrian #1 or Locrian #2 scale.
From a very serious source, of a very knowledgeable Musician, Don Kasper.
From here:
Anyone have a harmonic analysis of Stella by Starlight?
"For example, in the first measure, Emin7b5, try hearing and incorporating an F#(the natural 9) in that chord, as well as the following A7b9 (now with "13!"). That F natural in the Emin7b5 is not a pleasant note, IMO - the F#, is, to my ear, a note that should be heard and explored melodically, as well as harmonically. Abiding by the key signature and melody, only, may delay this discovery. Try it at a piano and see if you like the F or the F# in that Emin7b5 chord. In the A7b9 chord, the F sounds great, as well as the F#. Any experienced jazz pianist or guitarist (IME) would not play the F in the Emin7b5 chord, but might play either the F or F# (or no "F" at all) in the A7b9 chord."