I think it’s because they plan to put the M3 Pro chip inside an iPad Pro or maybe even the Apple TV - if they market it for gaming. Or maybe just trying to shrink the Mac mini but still keep the pro chip inside.

Either way, I think they’re trying to lower the TDP of the chip so they can fit it into smaller devices.

My assumption is each core with ray tracing consumes more power compared the cores without ray tracing. So their thermal budget likely took a small hit just adding the ray tracing cores to the M3. Meaning they could only bump up the GPU core count by a small amount.