I think the actual strength of S.American teams seems stronger if you compare the results , also between the top teams...
If we compare the last 3 World Cups, Europe has done better in head-to-head with South America. Only in the last World Cup did South America do better, and they had home advantage. Also, many of the wins came against second class European opposition (Greece, Bosnia, Switzerland, etc.). So in this case, if you actually look at the results, it seems exactly the opposite of what you say it seems.
2006 - 10 matches, South America 3, Europe 5, 2 draws
2010 - 10 matches, South America 1, Europe 6, 3 draws
2014 - 15 matches, South America 8, Europe 5, 2 draws
TOTAL over last 3 World Cups - South America 12, Europe 16, 7 draws
If we talk about top teams (Argentina, Brazil, Uruguay for South America, England, France, Spain, Netherlands, Italy, Germany for Europe), the balance is even more in favour of Europe.
2006 - 3 matches, South America 0, Europe 1, 2 draws
2010 - 5 matches, South America 0, Europe 4, 1 draw
2014 - 6 matches, South America 2, Europe 3, 1 draw
TOTAL over last 3 World Cups - South America 2, Europe 8, 4 draws