Maybe Apple will implement 2-core SoCs on their phones, 3-core SoCs on the iPad, 4-core SoCs on the iPadPro, and 6-core SoCs for MacBooks. The possible reasons are as follows:
1. Apple seem to work very deliberately, and do not over-specify their devices; e.g. 3-core SoCs for iPad, not 4-core.
2. They need to differentiate their array of products, and the SoCs are one aspect of this.
3. There is thought to be a market for larger tablets, for genuine professional use, hence the work with IBM; this would need extra processing power, hence 4 cores.
4. ARM and partners estimate the market for AIOs and Laptops at 250 million/year in 2020; they are focussed on addressing this opportunity.
5. Apple are the most advanced SoC designer, and are certain, in my view, to want to address this market themselves, and not leave it to others.
6. Intel's 6th generation mobile CPUs, such as i7-6600U (4 thread, 15W) and i7-6700HQ (8 thread, 45W) are the standard established by Intel.
7. A 6-core Apple SoC would compete at the level of i7 mobile.
8. Multi-threading is probably ruled out by the need to be a power-miser, in the usual Apple tradition; ARM enlarged on this in a 2013 paper.
9. Costs would be considerably reduced, compared with Intel processors.
10. ARM are coming with A72 (Maia) powered devices on 14/16 nm in 2016.
11. ARM powered devices using ARTEMIS, the successor to A72, should arrive in 2017, and ARTEMIS should be targetted on 10nm.
12. The successor to ARTEMIS will arrive one year later in 2018, in devices.
13. Apple have to move more rapidly, to stay ahead of mass-market devices, using stock ARM IP.
14. TSMC 10nm may be in full production by 2017, maybe enabling a 6-core solution.