Decided to look into game 1's since 1970 for U/O trends. 10 games, 7 under, 3 over. Only 1 game saw more than 3 goals.
2006 Germany 4-2 Costa Rica O
2002 France 0-1 Senegal U
1998 Brazil 2-1 Scotland O
1994 USA 1-1 Switzerland U
1990 Italy 1-0 Austria U
1986 Italy 1-1 Bulgaria U
1982 Italy 0-0 Poland U
1978 Italy 2-1 France O
1974 West Germany 1-0 Chile U
1970 Mexico 0-0 Soviet Union U
Generally premiere games in soccer (league as well) are low scoring. I would guess it's because teams are quite satisfied with a draw in those games and because you haven't gotten to know your team mates in a competetive environment.