As you can see, the latter part of the test completely maxes out all the CPU cores. It is clear that the benchmark is artificially creating a high workload, and since nearly all the cores are running at full speed then SoCs with more cores will score better for that part of the test. I never saw this kind of workload on any normal apps.
In one way it is the benchmarks which are artificially inflating the performance benefits of octa-core phones (rather than the power efficiency advantages). For a more comprehensive look at benchmarking check out Beware of the benchmarks, how to know what to look for.
Why are light apps using 8 cores?
If you look at an app like Gmail you will notice and interesting phenomenon. On a quad-core phone the core usage was evenly split between 2 and 4 cores, but on an octa-core phone the app used between 4 and 8 cores. How come Gmail can run on 2 to 4 cores on a quad-core phone but needs at least four cores on an octa-core phone? That doesn’t make sense!
The key again is to remember that on big.LITTLE phones not all the cores are equal. What we are actually seeing is how the scheduler is using the LITTLE cores then as the workload increases the big core are brought into play. For a while there is a small amount of crossover and then the LITTLE cores go to sleep. Then when the workload decreases the opposite happens. Of course this is all happening very fast, thousands of times per second. Look at this graph which shows the utilization of big vs LITTLE cores during my testing of Epic Citadel: