Yes, it is most likely relative absorptivity at the incident wavelength. Without knowing molecular structure or at least having an absorption spectrum it's impossible to say why. Essentially, all things being equal, if a molecule absorbs more light, it radiates a proportional amount via fluorescence. (Although, there are exceptions - the quantum yield can be wavelength dependent in some situations.) There is a resonance condition for absorption, and depending on the molecular structure (or solvent), the absorption bands may shift to higher or lower energy (or intensity). This is why molecular compounds can appear to be different colors by eye. So, the molecule in question absorbs better at the one wavelength than the other.
Raleigh scattering really has nothing to do with this problem.