This happens to me every 3. game I bet on basketball.
Just now, my team was down by 19 in the 1st half and 17 at HT. 2min before the end of the 3rd they come within 1 just to give up 8 straight again.
90sec before the end the make it a 1 point game again just to give up 9 straight, WTF.
Most teams don't want to win a game by 50 points. They want to pace themselves to prevent injuries. Save energy for future games.
If they're leading by 19 points, they'll relax and go easy. Do just enough to win.
Sometimes if they have a big lead they'll put their star players on the bench. Put the secondary or 3rd string in to gain experience and game time. Later if the game is close, they'll substitute their star players back in the game. Scores will see saw and fluctuate to mirror these adjustments.
This is very common in the NFL where injuries have become a concern. Even mixed martial arts fighters train only 30% to 50% of their maximum potential if they think its enough to defeat their opponent in some of the smaller events. Its common for UFC fighters transitioning to bellator to put less effort into training, believing they don't have to be at their best to win.