THE minor scale is the natural minor. That is the "default" setting for minor scales. The other scales are called altered scales because they are variations derived from the natural minor scale.

In strict traditional music theory, when you alter the natural minor scale to harmonic or melodic minor you change the scale going UP (ascending) but it returns to it's natural state on the way back DOWN (descending).

This is done because the voices lead up and then down with more half-steps creating more "leading tone" tonal resolutions.

This is the list of minor scales with formulas.

Natural minor:
1 (WS) 2 (HS) min3 (WS) 4 (WS) 5 (HS) min6 (WS) min7 (WS) 1

Harmonic minor:
1 (WS) 2 (HS) min3 (WS) 4 (WS) 5 (HS) min6 (WS+HS) maj7 (HS) 1
(Descending reverts to Natural Minor)

Melodic minor:
1 (WS) 2 (HS) min3 (WS) 4 (WS) 5 (WS) maj6 (WS) maj7 (HS) 1
(Descending reverts to Natural Minor)

Some sources do not note that the harmonic and, or melodic minor scales revert to natural minor upon descending. It is not required to revert to natural minor upon descending.

Historically, it was noticed that many Baroque & Classical composers (Bach, Haydn, Mozart, Beethoven, etc.) altered scales in this manner in their music. Therefore, it became the preference of music theorticians when presenting the scales in an organized, academic setting. So, it became the "industry" standard.

Hope this helps.