I don't think it has anything to do with that.
In my opinion, over the years teams have become more and more even. Its not like 20 years ago where you had your 4 clear candidates and the rest were just extra teams.... now you play a World Cup and you have excellent teams like Argentina, Spain, Brazil, Netherlands, Germany..... revelations like Ivory Coast, Chile, Paraguay, Ghana, etc. Therefore games are just getting harder because the level is getting more and more even.....
First round is more often than not the most important one. Why? because this sets a team not only up a table (a first game win means that you can potentially get through the group with a tie on either of the two games you have left) but it also sets a team MENTALLY. A loss on the first game brings really brings a team down, because they realize they must win one or both next games.
Therefore we see many great teams playing more conservative. As an example, if you watched the game between Argentina and Nigeria, you could see how a much better team, in this case Argentina, decided to just hold on to the ball once they got that goal. The first 10 minutes of the game were quick and you could see Argentina pushing hard. After the header (which turned into a goal), you could see the pace of the game slow down A LOT. Argentina wasn't really trying anymore, and the ball would constantly end up back in Argentina's side with the defenders just passing it between them.
Its a strange strategy imo, because one goal from the opposite team could make things different....
Once again, expect to see teams on the second round score more goals.
Teams that need the win will push forward and therefore will either score more goals, or leave the defense wide open for counters...