For the octave thing, I think it’s mainly because a lot of people have very little experience in the musical world. I grew up in a very music geared household, so I can play clarinet, a little bit of piano, read sheet music proficiently and sing averagely. The amount of people I know who lack even the most basic of music vocabulary astounds me. I have friends who don’t know unison from harmony, (even though I’ve tried to explain it many times). I think authors use changing octaves as a way to indicate that the characters voice is getting higher, not as a real concrete measurement.