Data source: NCEP CFSR Reanalysis data from 1979 to March 2011 using T382 data assimilation similar to NCEP GFS model prior to 2010 upgrade. Data after March 2011,
called CDASv2 extension which uses T574L62 data assimilation with output on 1760x880 gaussian surface grid.
This is NOT station-data but actually a form of weather forecast data (short-term) of 0-6 hours assembled using state-of-the-art data assilmation techniques. Not only are station data included, but all other forms of available meteorological data including satellites in order to product the best, most accurate depiction of the entire climate system.
One can expect differences w/ NCDC station data as individual locations on the order of 1,000s may not be representative of 1,000,000 grid points in model depiction.