This is because mediated has bigger caches in its arm CPU implementation. And sure, higher frequency. But in the end, those extra megs of cache do make a difference.
The Dimensity 9000+ is clearly the better choice for non-gamers as the CPU is more important than the GPU and even for gamers the difference in GPU is negligible in every game that isn't Gemshlin since the phones usually sit locked at the max frame rate anyway and only synthetic benchmarks show any differences.
AnTuTu has now released its performance rankings for the month of September. The Asus ROG Phone 6D leads the pack, in no small part thanks to the MediaTek Dimensity 9000+ and its class-leading CPU performance.