The basic concept is that everyone is fresh and has maximum intensity in the 1st quarter - everything is contested, everyone is highly alert.
As fatigue sets in, and offenses adjust, then you see the scores rise.
I think it is even more than just that.
At the beginning of the game the shots are taken only considering the offensive skills of the attacking team.
As time goes on the attacks are adjusted to the defensive skills of the opponent.
The adjustment concerns both the shots distribution among the players and the shot distance for each player.
If your best shooter is opposed to an even better defender, he will probably shoot and miss a lot at the beginning of the match, and then the team will start looking for better offensive solutions...
The game engine is really smart :)