It's not a random guess - it might surprise you just how low the yield actually is on CPUs.

Yield is inversely proportional to die size, so the larger the die, the lower the yield. Yield is also lower on newer manufacturing processes. Both apply here. Furthermore, although some chips might be only marginally defective and 'binned' to a lower SKU that option isn't open to Sony. They have no use for those lower-clocked chips.

Chip manufacturers never publish their yield figures, but if you look around you'll see yields of around 60% are certainly not uncommon or unheard of.