Even though Apple equips both the new iPad mini 6 and the iPhone 13 with its quick A15 CPU, differing clock frequencies lead to a small performance difference in the single-digit percentage range that should only be noticeable in synthetic benchmarks.
https://www.notebookcheck.net/Underclocked-The-A15-chip-in-Apple-s-new-iPad-mini-6-is-slower-than-in-the-iPhone-13.562249.0.html
An easy explanation would be the use of a lower bin silicon in the iPad. In which case it's not guaranteed that it uses less power just because the frequency is lower. It can be less efficient. The iPad surely has a bigger cell and so it can tolerate it better. Similarly, the 13 and 13 mini could be using chips with a faulty GPU core. Improving overall yields for Apple.