Although there are only seven alphabetical names used in music - A B C D E F G - these notes are repeated on higher and lower pitches over a large range of notes. Because everybody has a different voice range, not all singers will be comfortable with the same pitches.
The actual pitch of the notes given in the examples in this book are just a guide, so if you feel comfortable singing the examples on a higher or lower pitch than the ones given here, that’s fine. The important thing with these examples is to get the correct timing.