I don't really know what to expect with an X20 but I certainly wouldn't expect performance to scale with cores like it does on server and workstation CPUs.
This is almost all down to heat dissipation. A server chip is typically attached to a big heat sink and the resulting thermal system is designed to draw sufficient heat away from the CPU to keep it running at its full speed. A phone chip is different. Speaking approximately, it is given a temperature limit and the CPU is allowed to run at full speed until the limit is reach at which point it will regulate the processor speed to maintain the SOC at or near the temperature limit.
So when you run benchmark on an X20 the scheduler and the thermal manager will be doing their best to get the best performance possible from the silicon, dynamically tuning things based on temperature readings and estimated CPU utilisation.
If these dynamic tunings were perfect (they are not and will not ever be; they can be good but they will never be perfect) it would be impossible to improve the GeekBench performance by enabling more cores precisely because the dynamic tuning would already have selected the right number of cores to maximise performance.
Or put another way, if you did see a significant performance uplift by turning off PM features (like automatic CPU hotplug), that would typically evidence of a badly tuned base system. It would not be a good thing!