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.

Symmetric doesn't mean 'power of two'. Quite a few ICs have had even numbers of cores on them (I'm not aware of any with an odd number of cores, but that doesn't seem like the end of the world either). Lots of other things have to fit on the die, too (e.g. unified cache, which takes up a lot of real estate in chips that have them)

slightly off topic, but for some reason your comment made me remember the time I had to do some regression testing of a fix backported to a really old version of our software.

We had to test against the OSs we'd since dropped support for, which included HP-UX, AIX and s390.

The first time I heard someone mention a "31bit" build, I thought they were taking the piss :P It just seemed so odd at the time...