For the most part once balls drop below 11-12 pounds. The "High Tech" cores don't make that big of a difference, so they just use the "generic" cores.
I believe also it has to do with the densities of the materials used to get the lighter weight. The most important thing is the coverstock.
There are a few balls that have a high tech core in the lighter weights, but you have to really research to find just which ones do.
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