The source/filter model of speech production says that the complex wave that actually comes from a speaker's mouth depends on two things:
The frequency response curve shows how the vocal tract in neutral position would respond if you gave it various frequencies:
The spectrum of the glottal wave shows what frequencies you're actually giving it:
Putting these together gives you the spectrum of the wave that comes out of the mouth for a schwa:
The two aspects of the source/filter model are independent of each other.