I'm pretty sure its got to do with the more levels you complete at a higher difficulty, the more SPI is accredited to your total score.

Thank you. I will keep trying.

It is very competitive, I seem to lose SPI on bad or not enough effort rounds.🎶

I'm pretty sure you don't actually lose SPI. It seems to be based at least somewhat on your top three scores for each game.

As far as I can tell, SPI is solely a function of 36 numbers which are the top three high scores that you have achieved in each of the 12 games. Each level is more difficult than the previous level, but awards you with more points for playing in it. So if you play a game at level N and achieve high scores X, Y, and Z, (with X the top score) and then break through and achieve level N+1 with a score of X+K (where K is small), then your high scores will by X+K,X,Y and your SPI won't go up by much. But if you play the game several more times at level N+1, you'll get three new high scores that are much higher than X+K,X,Y, and your SPI will improve dramatically. So one corollary of this is that, if you want your SPI to go up, whenever you achieve a new level, immediately play several games at the new level.

Also, it's important to know that the number of points awarded at each level goes up non-linearly (maybe exponentially). It seems to me that it's about as difficult to get from an SPI of 300 to an SPI of 3,000 as it is to go from an SPI of 3,000 to an SPI of 30,000. It took me a few months to get to the Gold Ears.