They measure the chain at the LBS (a Trek dealer in Montevideo, Uruguay) with a park measurement tool. And I got 1350 km (about 850 miles) on my first original chain (Shimano), and about the same on a PC-58 SRAM chain. So I don't think I am using the wrong chain.
I am on the third chain (another PC-58), which has about 200 km (about 125 miles) on it.
I usually ride on pavement (on the road), I rarely ride on dirt, mud, etc.
When I bought the bike, they told me the chain would last an average of 1200 km (about 750 miles). According to them, that's the average life of a chain.
I don't think they are telling me this just to sell me a chain, because they have a very good reputation here.