I suspect Apple was being aggressive on cooling designs based on inaccurate TDP information from Intel. Intel just made the power/thermal footprint configurable - so when you got the final silicon, there was a decision between lower performance than promised, or higher TDP than desired.

Multiply this by adding the chipsets, other IO controllers, and discrete onboard GPUs.

I suspect Apple are being overly conservative now, partially because these chip designs are new, partially because no internal team wants to be where fingers point if the design is inadequate, and partially because a SoP design gives them a bit more space/flexibility to do so.