Because, the dot gain can change radically when you change your vinyl supplier. That's because the dots are too small, and too much of the white background shows thru between the dots. If, on the other hand, the cheaper vinyl has too much dot gain, the colors will bleed together and the sharp edges will look terrible. Generally, vinyl tends to have too little dot gain - while paper tends to have too much dot gain. It's easy to see why.
Paper is more absorbent than vinyl, so the ink spreads out farther before it dries.
Vinyl is not absorbent, so the ink tends to bead up. We try to make inks that have the very best dot gain for their intended use. For vinyl, we add special solvents and wetting agents to help the ink spread out. Wetting agents are materials added to lower the surface tension of the ink - which allows the ink to wet the vinyl. Only a small amount is needed to act as a bridge across the interface between the ink and the surface of the vinyl.