Comment by JyrkiAlakuijala

I made a mistake there with subtract green.

If I had just added 128 to the residuals, all remaining prediction arithmetic would have worked better and it would have given 1 % more density.

This is because most related arithmetic for predicting pixels is done in unsigned 8 bit arithmetic. Subtract green moves such predictions to often cross the 0 -> 255 boundary, and then averaging, deltas etc make little sense and add to the entropy.