Was wondering exactly the same about the 'il nostro' or not, in this case. Have just come back to DL having had no internet for nearly two weeks - very aggravating - and I started near the beginning again to see how much I've remembered. Not as much as I'd hoped unfortunately!
In real life when you talk about "him/her" generally the person would be around and maybe you would be pointing at him.
For example, when you say "It's his car". The owner of the car would naturally be around and you would use body language to tell who are you referring to.
In a book/newspaper, the rest of the context will clear who (him/her) are you talking about.
Good one. In Portuguese you have a similar construction that matches the object (le mele sono sue <=> as maçãs são suas <=> the apples are his), but you can use a different construction as to match the gender and number of a 3rd person noun (as maçãs são dela <=> the apples are "of her"). Wonder if there is such a thing in Italian.
I think i cracked it. È il nostro coltello. = It is our knife. Il coltello è nostro. = The knife is ours. In the first sentence, the knife is passive, while in the second it is actively doing something (being in this case), therefore the different translations of the possessive determiner. I hope that makes sense.
Note the difference: Il coltello è il nostro - most common, neutral statement of fact - the knife is ours, the police asked, so I am explaining.
Il coltello è nostro is marked, emotional, stressing the ownership - è tutto mio - it's all mine, special, my precious :-) Also look at this headline: Il libro è mio finché lo scrivo, i.e. The book is mine while (for as long as) I write it.