Reaper paints can be confusing. The various lines, combined with samples and limited editions and specials, change relatively frequently ... with lines being phased in an out and specific colors becoming available, then not. And, as noted, the differences are not presented up front.
I have made my peace with this phenomenon by simply treating each bottle as unique. I don't concern myself with the line and its density and its additives; I've learned what density I like to work with, put a few drops on my palette, add my standard additive mix until the paint is at my preferred consistency for a particular application, and then paint away.
It becomes all about the colors. Which are pretty great with Reaper paints.