I thought I've seen reports that the reason it's a 7-core chip is due to chip binning - basically, all chips are 8-core, but those that don't pass all QA tests on 8 cores go in as 7-core ones.

The pricier M1 iMacs, on the other hand, deliver an 8-core CPU and 8-core GPU. M3 iMac users should see an uptick in graphics performance –– and not just because of the extra GPU cores.

The M2 Pro uses a second-generation 5nm process, and offers more CPU and GPU cores, memory bandwidth, and several other upgrades over the M2 and M1 Pro chips.

Like its predecessor, at the core of everything the M3 chip does is power efficiency, and the M3 CPU and GPU can deliver the same performance as the M1 with half the power.