I'm not sayign its' not possible. My downstairs is 38-48% withotu even slowignhte blower when it's above 80F outside. I'm jsut saying that in some homes it's not possible. In my last home ,the unit was oversized and even running the 2 stage scroll in high stage with low airflwo, and using 350CFM/ton on top of that helped a LOT but still didn't keep it all that dry. Infiltration was also a major factor in such a small home.
The smaller the home, the greater the surface area to volume ratio is. So infiltration is comparatively greater. Same with a single story home, vs. 2 story.