What is the definition of the "solar mixing ratio"?
An answer which explains it possibly, but remains speculation as I don't find the term explained:

Taking the pristine abundance of atoms, from which the sun formed (thus solar mixing) gives you a ratio of H and He atoms of 92% hydrogen to 8% helium. Converting that to the place and phase where we have molecular hydrogen (H2), gives a molecular mixing ratio of 83% hydrogen (H2) and 13% helium (He) - which is pretty close to the values given there. They probably use another source with slightly different numbers for the pristine elemental abundances than this wikipedia article.

Thus I consider it likely that they compare molecular abundances. They compare the abundances or ratios you would see, if you took the pre-solar nebular without any chemical segregation and formed the giant planets with the one measured by their spectrometers when actually looking at the atmospheres of the planets.