It's not about _qualified_ runners, it's about the size of the total field accepted.

Boston allows "roughly" 25k participants, but that number fluctuates somewhat every year. If they allow 30k, the cutoff delta goes down. If they allowed an unlimited field, the cutoff delta would be 0, and you'd only have to worry about your published qualifying time.

The slider adjusts the number allowed to run (apparently the pool of time qualified candidates can be assumed to be larger than the max value of the slider).

It's a cutoff delta, you have to subtract it from the "preliminary" qualifying time.