On Say You Say Me he starts out singing in chest voice in his upper mids.
E4 G4 A4 E4
Say You, Say Me That has a chesty timbre to it.
On Creep, he starts out singing about an octave lower. That's just automatically going to have a different timbre to it than upper mids around A4.
G3 A3 B3
I'm A Creep This is much lower and sung in a deeper placement.
In the chorus on Creep, he then goes into an airy falsetto head voice, then shifts into chest for an
F4, F#4, G4, A4. He leans into the chest part of the chorus.
This is a stylistic choice type of thing. Say You Say Me by Lionel Richie had a completely different vocal approach. Lionel did not lean into the sound at all like this.