AndrewH said
a lot is simply open to interpretation.  

...because notation is inadequate - the composer has an idea, uses what limited notation is available. Then the composer dies. Then someone has to fill in the gaps, i.e., interpret. It's the same with poetry and drama.

The best you can do is if the composer has conducted an orchestra or played the piece, and been recorded, but that's not really any kind of gold standard, as tastes change and some composers can have bizarre ideas that were worth amending. Some composers wrote for inferior instruments, or ones that differed greatly from modern instruments.

A further orchestral consideration is that no notation has ever existed as to how each section should counterbalance the other sections. That's always the conductor's vision.

Originally pp meant pianissimo, which is Italian for as quiet as possible and ff meant fortissimo, as strongly as possible. During the 19th century composers started writing ppp, pppp, ppppp, fff, ffff, fffff, which are logical impossibilities, but keeping up with the industrially improved instrument technology and possible balance of orchestral sections was perhaps intended. Or perhaps they wrote for orchestras that had become lackadaisical and lucklustre through playing too much Haydn?

As a footnote to this discussion, I'm reading Hilary and Piers du Pré's biography of Jacqueline and I noticed this quote:- (Jacqueline Du Pré) "Once the composer has written the music down, it no longer belongs to him, it belongs to me."