Vocal range predictable???

It depends on how developed their voice is. But you know, it's surprising because I have this friend (mezzo-sporano) and it's weird because her speaking voice is soo different from the voice that she has when she starts to sing. it's amazing. you wouldn't guess she had that kind of voice. anyway, for males, i guess it's easier because they have a certain timbre in their voice. but still, it's different for different people.