If you don’t see a difference, you are already very close to the optimal correction and a subtle change in either direction is unnecessary. They will still go ahead and try fine-tuning the prescription and when you tell them that A and B are the same, they can fall back when they add another correction. A and B might currently be the same, but if you add C, A+C could be better than B+C.
That is not a horn. Purple correctly identified it as a trumpet.