Just my personal opinion;
You have to look at the entire reason for a design ,and intent. There is a high probability that lighter weight balls are being used by people who are going to be throwing the ball "slower" than average. In the prop shop industry, it is more often than not, that slower speeds tend to have better ball reaction with less aggressive core numbers. So with that said, it makes sense that lighter weight balls will use cores that have higher Rg's and less differential built in to it.
I hear ya and agree with you about slower speed people. However, In my case, I throw 12# harder than 14# & 15# and throw 13# harder than 14# % 15#, but not quite as hard as 12#. Thus, I need aggressive core numbers. I don't throw 12# & 13# balls because I am 78+ and an old man. I throw those light weights because of back problems.
I just had my second back surgery last week, having had my first one 22 months ago. The Dr told me 10# on bowling balls, especially considering that I may have to have 2 more vertebrae repaired in the near future. I fussed with him and he finally told me 12# and no more. 9 months ago, I sneaked up to 13# and now I am out again.
To sum it up, were not for my back problem, I would still be throwing 14# & 15# bowling balls @ approx 13-14 mph. With 12# it is more like 14-15 mph or a wee bit more, thus the need for aggressive cores.