This is likely going to change as software support for gaming on Linux improves.
If you consider real high performance computing, with well optimized libraries that can properly use the hardware (including GPUs), 50 % difference between windows and Linux is not really surprising. This is the reason 100% of real high performance computing is done on Linux. It is a better OS for raw performances than windows. For some tasks we are easily talking over twice the performances. It is not always the case, but not surprising at all.
The differences clearly depend on the actual low level implementation of the code. But in general the current situation in gaming, with windows that competes with Linux on raw performances, is only due to lack of software support for gaming on Linux. As this is changing over time, we’ll see games performances greatly improve in Linux. Hopefully until the physiological surpass of windows performances.
Currently most of gaming support on Linux is done via some kind of translation layer, that has itself an overhead. It means that the real linux performance would be even better than in all these benchmarks, if it was really possible to compare 1:1 Windows and Linux with native, well optimized code.