Dual layer capacitive sensor in PCB better?

[nielsfaber]

I see that the V2 PCB layout uses copper plates on the top layer for capacitive measurement.
I would expect that copying the same copper plates also to the bottom layer will increase the capacitive coupling by approx. 2 times.
Hence the signal-noise ratio of the measurement becomes much better.
Is there any specific reason why the single layer is used instead?
Am I overlooking something?

[nurikk]

It’s good enough to use current traces, moreover changing tracks will require recalibration process (quite sophisticated and undocumented)