Sizing capacitor across op amp inputs
I agree with Andy. although there is a clue in the schematic.



Note the 18pF capacitor (which partly forms the internal compensation pole, amongst other things). It is possible that the original design had some instabilities due to ripple / noise on the 5V input and this capacitor was added. The actual external pole is going to be quite a high frequency (as it is dominated by VR19, when looking at the resistance seen at the inverting input, VR19 || R28).

Using the same value across the inputs as the internal compensation capacitor is stretching the limits of coincidence, so it was probably chosen for this reason.

Just why is very likely lost in the mists of time.

I used the answer box so I could add the image.