Could you give me an example, just to illustrate? I don't mean to be a pain, but I am curious.

Some background: I'm looking at a Burley Sand Point. One on-line store replaces the original rear derailleur, a Sunrace, with a Shimano Alivio before they ship it. This raises the price from $750 to $799. If I buy the Alivio, how will it likely perform better than the Sunrace? Is the Sunrace made of cheap plastic, while the Alivio is made of chro-moly steel? Is the Sunrace an old, less-competent design that has been sitting around the warehouse forever, so they're selling it cheap to get rid of it?

I'm trying to become more knowledgeable about bikes, partly so I can purchase wisely, but partly because I'd just like to know more.