It appears to be either company size or US didn't use the latest reporting format for exec compensation.
"The 500 companies in this database were selected for two reasons: first, they all reported their 2006 compensation information following the SEC's new compensation disclosure requirements, which took effect for companies' Form 10-Ks for fiscal years ending on or after December 15, 2006; and second, they are all among the largest companies in the U.S., measured by market capitalization and/or revenues."
If you click on the "Technical Info" tab there is a list of all the companies that are included. US is not among them.
Jim