Having one core dedicated to coordination tasks and maybe to get all interrupts when other 8 are churning things is always a good idea from my perspective (HPC admin and programmer).

Also Apple is not shy of odd numbered cores. iPad 2 had a tri-core GPU for example.

Some models of the M4 iPad had 3P + 6E = 9 cores as well, so it's certainly not unusual. Like another commenter said, the "why" can be binning for chips which come out with a broken P core.

AMD used to have a 3 core desktop CPU. IIRC it was because one of the intended 4 cores was bad but they could salvage the package by making it 3 cores. The 3 core option was so popular, that they kept it once the 4th core issues were resolve with a software patch. Clever people figured out how to enable it once installed and got a 4th core for free. Never used it myself, and only heard tales about it. Could be urban legend.

Nothing magic about even or power of two core counts.

https://en.wikichip.org/wiki/chip_multiprocessor

Lots of "odd" counts by your two criteria (though not many odd counts).