Stephen Schwartz had envisioned Kristin Chenoweth as G(a)linda as writing her, but had written the vocal range for an alto/mezzo-soprano. However upon Kristin’s agreeing to the part, they changed her vocal ranges.
Whenever G(a)linda is acting as though she is perfect and good, she uses a classical soprano voice.
Whenever G(a)linda is being herself, she uses more of a mezzo-soprano voice.
So that’s why in For Good she sings lower notes. Woah!