It is my understanding that Motiv uses in each ball line up, the same size and shape core in all weights with the lighter ball cores being less dense. They do this to give the lighter weight balls as close as possible the same shape on the lanes as the heavier weight balls. That is what I have been told by Motiv themselves.
Other companies such as EBI, Storm, Brunswick, etc. use the same generic core in all of their lighter weight balls which is a different core than they use in their 14# and up balls, so their lighter weight balls aren't going to have the same performance/shape on the lanes as their heavier weights. That is the main reason I try to stick with Motiv plus Motiv has great and long lasting covers that don't soak up oil like their competitors do.
However, as already mentioned, I have not done as well with their lower differential balls.