I know many aren’t going to believe my story, but some months ago, when I was trying to adapt to the 13 mini, I tested several iPhones. One of the minis had, in Geekbench, the same metal score as the 13 pro. So my guess is that the regular A15 was a binned version of the A15 with 5 GPU cores, and I got one that didn’t have the fifth GPU core disabled. This unit was manufactured in may or so, unlike the one that I tried in Christmas that had, effectively, just 4 GPU cores enabled (lower metal score in Geekbench 5).
I’m not sure if I still have the screenshot with the GB metal result but it was on the 13 pro league. Only 4GB of RAM unfortunately.
I ended up returning it because the screen was very uncomfortable to my eyes.
TL;DR: yeah, I tested several iPhone 13 minis, and the last one I tried had a 5 core GPU.