The OECD's methodology is that they publish the numbers as they are provided by the member states and/or as they are available from public sources.
What the OECD publishes is either the broadband penetration numbers they receive from the nations themselves (FCC, department of commerce etc provide those numbers for the USA) and they look at the offers from the largest operators in each nation to be able to do a price broadband comparisson. For a good explanation of how this works see the explanation Taylor Reynolds of the OECD provides. http://www.itwire.com/content/view/12224/1154/
What you are doing is you measure the throughput to a US site (Speedtest) from Korea and then compare this with the numbers as given. In doing this you forget that trans-pacific traffic is still a bit expensive and therefore often rationed. So while Koreans may get 50mbit/s on the network of their provider, of net in Korea this already may be lower, but towards the US this is definitely lower. Furthermore and on top of this, Speedtest relies on people going to their website and testing the speed. It just might very well be that Koreans are not too interested in the speedtest website, seriously affecting its usability.