Beginning students in improvisation find that even though they are improvising within the correct key center, their melodic lines do not create a sense of tension and resolution cuz they are not hearing the implications of chord progression.
As one grows musically, the ability to improvise inside the chord changes and define the harmonic structure of the song deepens.
Professionals take the process one step further — they hear the chords in their heads but choose to play outside of the harmonic structure. This approach works well when the improvised lines hav a strongly defined rhythm and periodically touch base with important chord changes such as the V-I relationships. A discreet listener will hear the dissonance as intentional and the results can be very exciting.