Is it caused by the UV light, heat or both? I've seen the Sam carbonization effect with CFLs, but not with incandescents which run hotter.
The heat of the incandescents makes the grease from the dirt to evaporate before it touches the bulb, so the residue is then less sticky. Plus the air moves there faster, so it blows tge dry residues away.
With fluorescents maybe tye bare tube is able to evaporate the grease, but the reflector is colder so it condenses back there.