Uneven belt tension graph

I've had this problem for a while now where I can't get the second peaks to overlap anymore. I've already done a lot of rebuilding, but the behavior remains the same. On image 1, I roughly tensioned by ear so that both belts sound about the same. On image 2, I tensioned by 2 turns. It doesn't go up, but instead a plateau forms. The belts are identical in length; I removed them and checked on a long table. One was one tooth too long. Since then, I've rebuilt the y-rails, both steppers, HD Idler, and a new print head was added as well. But that didn't help either; the behavior stays the same. It prints quite okay, but it used to be much better. Does anyone have any idea where to start?