"Runniness" is called viscosity. Viscosity has nothing to do with quality of paint. You can lower viscosity (increase runniness) by adding diluent (which for "latex" paint is water). In fact, you are required to for spraying; you must adjust viscosity according to sprayer instructions. Viscosity is measured with a Zahn cup.
I don't know what paint manufacturers do to make their grades of paint different from each other. I suspect it's the UV resistance or toughness of the resin (the "glue" that holds the paint together; it does not evaporate and is not pigment). Bases than "Deep Base" contain large amounts of white pigment. It's essentially deep base + a whole bunch of white pigment already added, so the formulas for pastels and off-whites aren't insane. If better grades give more white pigment for better coverage, that would mean the paint formulas must also change. So ask your paint supplier about that, but I doubt it's the case. There certainly are different qualities of pigment but each grade requires having a different paint mixing machine with different pigments in its carousel, and big-box stores ain't got time for dat :) So you're just getting cheap pigments in any paint you buy there.
That's fine for black and white pigments (carbon black and white titanium) which are cheap, legal and durable. The trick is color pigments. Now you know why cars are mostly shades of gray and houses are mostly pastels. Color costs. Stable color costs a lot. I use top shelf marine paint and I was once surcharged $200 for red pigments over the cost of black. The color held, though!
You shouldn't be using "more coats of topcoat" to try to cover up varying color or texture shades of substrate. You should be using primer for that, until the surfaces are more or less equal, before you topcoat. Why? Primer is cheaper. (and does not have the UV protection or toughness of topcoat). And it results in better finish because all surfaces are more or less equal before paint begins.
