Male/Female Key vs. Octave

Question

In many pop songs today, when the song is sung by one gender:

• The same gender as the singer would sing it in the original key and register.

• Another gender would sing the transposed version written in a different key that is usually a 5th away from the original key.

But why don't they do it like this? (I've always been taught to use this method, and it works completely fine to me as well.)

• The same gender as the singer would sing it in the original key and register.

• Another gender would sing in the same key, but an octave lower (or higher).

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It is nearly unanimously received that women's voices are about an octave higher than men's, while men's voices are about an octave lower than women's. In fact, as a fan of music and a critic, I'll have to say that it is really strange that they use the "male/female key" method and not the "octave" method. I'm going to record the voice types as explained on a related question:

• (F) Soprano: C4-C6 | (M) Tenor: C3-C5

• (F) Mezzo-soprano: G3-A5 | (M) Baritone: G2-G4

• (F) Alto: F3-F5 | (M) Bass: E2-E4

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Disclaimer: This question may look like an off-topic one, but it is not.

Answer 1

The reason is the different vocal techniques used by women in classical and pop music.

Male singers mostly use similar techniques in both genres. The higher part of their range is preferred, particularly in solo singing.

In classical music women sing in head voice which fits naturally about an octave above a male voice.
The most important difference in pop/rock/jazz is that female singers generally sing in chest voice (i.e. in the lower part of their total range) and that often happens to be about a fifth away from the key that works for a male singer.