Due to wavelength-dependent optical attenuation in the skin, the local fluence at a subcutaneous vessel varies with the optical wavelength in a spectral measurement. Hence compensation for such a spectral attenuation is necessary in quantitative measurements of the oxygen saturation of hemoglobin (sO2) in blood vessels in vivo using photoacoustic (PA) imaging. Here, by employing a simplified double-layer skin model, we find that although the absolute value of sO2 in a vessel is seriously affected by the volume fraction of blood and the spatially averaged sO2 in the dermis, the difference of sO2 between neighboring vessels is minimally affected. Based on in vivo experiments, we demonstrate that the difference in sO2 between a typical artery and a typical vein is conserved before and after an experimentally acquired spectral compensation. This conservation holds regardless of the animal's systemic physiological state.