Therefore, a team like the USA may find it easier to accumulate ranking points as they play agaisnt weaker opponents in their World Cup qualifying group, CONCACAF and Gold Cup competitions - and are more likely to win more games than for eg the Czech Republic - who are likely to be playing against better teams such as France, Italy, Holland, England and Spain etc more often.
So the placing of 5th for a team such as the US in the FIFA rankings is pretty much misleading - this is probably why Mexico's rating is quite artificially high too - they always qualify for the World Cup because their regional qualifying group is probably weaker than that of a lot of teams in Europe, Africa or South America...
All of this explaing why the Europeans and South Americans take the FIFA rankings with a pinch of salt and igonre them in the main...