If i remember correctly, it was a binning issue. Some stupid high number (something like 50%+) of the new chips had issues in two of their cores, so they disabled them in the first run. When they refreshed to the next phone/laptop the following year, they had cleared up the manufacturing issues and turned on the cores, and then boasted about performance gains.

Somebody made a way to turn those cores back on, from what I read, but there was a chance you'd immediately glitch on boot and have to reinstall.