I have no direct knowledge on IBM's processor, but from working on other processors, usually it's due to product yield. Today's cutting edge fabrication of mul

I have no direct knowledge on IBM's processor, but from working on other processors, usually it's due to product yield. Today's cutting edge fabrication of multi-core chips makes it quite unlikely to get many dies to manufacture where all the CPU's are working at high speeds. Chips must run at a single frequency, so the slowest frequency is the bottleneck. If you fabricated 32 cores, and 1 fails testing, the chip is garbage. So the strategy is that the chips are fabricated with 32 cores, and then they speed-test the cores and take fastest 24 cores. The remaining 8 are fuse-disabled, and the end user sees a 24 core chip. Note this strategy also allows them to produce a 16-core chip with the same die, if for example only 20 cores work at high speeds. So you get both high yield and high speed chips.