If best way is the only...why do we see different setups ?

[abzoluut]

I see that, but, unless you're competing for world first, it doesn't matter that much, in the end.
Especially compared to how "fresh" it would feel to have more jobs, or more customization.

True. However, when you do the encounters a couple of weeks later (better gear and what not), it does not mean the "best" way isn't best anymore. It just means that people will find ways to make up for shortcomings a certain job (or setup) has. Fact remains that the best setup would have been better.