I don't think it is so much efficiency of their chips for their hardware (phones) so much as efficiency of their OS for their chips and hardware design (like unified memory).
See Lunar Lake on TSMC N3B, 4+4, on-package DRAM versus the M3 on TSMC N3B, 4+4, on-package DRAM: https://youtu.be/ymoiWv9BF7Q?t=531
The 258V (TSMC N3B) has a worse perf / W 1T curve than the Apple M1 (TSMC N5).
Dieselgate?
Also, there's the obvious benefits of being TSMC's best customer. And when you design a chip for low power consumption, that means you've got a higher ceiling when you introduce cooling.