The WRV list updates on the 1st and 15th of every month (real time). Each stat for a nation will either increase or decrease by one block, or remain the same. The change is random.
Click the 'WRV' link at the top of the page and it will show the WRV rankings for all nations. You can click "National Style Rankings" to view the rankings for individual stats or click on a nation to view all the stats for that nation. USA is currently 8/10 in Wrestling, Sambo, Jiu Jitsu, and Greco Roman.
It's mostly from having max level equipment and strong trainers. I typically cut fighters with low desire (poor sparring and weightcutting). Also, my nation is very high on the WRV for Wrestling and Greco Roman, meaning I have better chances of getting recruits with higher than average starting stats in those areas.
BH MMA Camp