Stuff you have built just because you can

When I still worked in the research facility which was before Apple moved to the IBM architecture, the major difference was that IBM had the latest manufacturing technology and Apple had a much better optimisation which partly had happened in the research facility.

So IBM had more power by finer structures and could push more MHz, the Apple chips were more crude but more optimized so they could do more with less MHz and you could push more MHz on the old chip structure.

For example, if the routing between the transistors is more optimized, you get shorter signal paths and all paths have a similar length so you can increase the clock without making the parts in the chip going out of sync or loose clock cycles.
Another problem is if you have 3 wires running a distance in parallel and the outer two change charge, capacitive coupling will make the line in the middle switch as well if you use too much energy to switch the outer ones. Using less energy prevents using more MHz. With special optimizing you can prevent that there are 3 signals running in parallel where only the outer ones switch so you can push a lot more MHz on the same technology.

That's why the Apple chips were highly reliable (older, fully developed technology) and still faster doing the same tasks.