All music is unique. It can't be sorted into genres. Genres do music an injustice.
Are all pieces of rock music identical?
No, obviously they're not.
But they have enough in common to be classified under a shared label, which serves as a convenience. It tells you a lot about the music so then you'll know what someone means when they say they like 'rock', but knowing that a song falls under the 'rock' genre will not mean that you know its every note before you hear it.
You'd never mistake a piece of Rock music for Classical, though (even if something like 'symphonic rock' has 'classical influences').
If you don't like the music genres analogy, consider animal classification. There are all kinds of animals, and it can be difficult to classify them on a species level. However, words like 'mammal', 'reptile', 'fish' and 'insect' allow us to convey a lot of information about their bodily structures without being specific.
Dogs and bears are both mammals, but this doesn't make them identical. Fred and Bob might both be melancholic/choleric, but this doesn't make them identical either.
It is possible for something to be classified under a label without it being any less unique.
We are all unique individuals, and the temperaments in no way attempt to undermine this abundantly clear fact.
Two people with identical temperaments probably won't be very similar people at all, as their upbringing and life experiences will have affected them in very different ways.
The temperaments attempt only to give convenient labels to a single *facet* of a person's personality. They make no attempt to define a person wholly.
Temperaments are labels that conveniently convey complex concepts concisely.
The words 'conservative' and 'liberal', for example, say a whole lot about the people they're used to label, but nobody would suggest that they define a person's entire personality. It allows you to say 'I am a liberal' or 'I don't like conservatives', and people will know what you mean, rather than having to list all the political views you like, dislike or hold in an exhaustive and tedious way.
It may interest you to know that while every snowflake is unique, they too can have their shapes classified into a handful of different general types.
This isn't exactly a misconception as such, but some people do further research about the temperaments and stumble upon the Five Temperaments concept, which adds the 'Supine' temperament to the mix.
I don't use this as I don't feel it's necessary. 'Supine' becomes the name for the Phlegmatic described on this site, and the name 'Phlegmatic' goes to a different 'middle of the road' temperament which is similar to the Phlegmatic/Choleric or Choleric/Phlegmatic blends.
With blends, it's unnecessary to have five temperaments, as the number is effectively brought to twelve instead.