Hiya

i have only recently been having the same discussion with someone else and i think we came to the conclusion that this particular robo just seems to be a little lazy and a little scruffy and just doesnt seem as bothered about how he looks, lol - so i think your robos is probably the same. Like you say, she is still eating, drinking and doing everything as normal so she isnt ill or anything and she has a sand bath, but maybe she just doesnt use it as much as she should!
Having greasy fur can be a sign of stress, but usually when the hamster has cage mates and is either being bullied or scared by the others - but your hamster lives on her own so it cant be that.
I have also heard that some robos produce more of the grease in their fur, so maybe your hamster has this problem. There is no treatment and it doesnt cause any other problems but it just makes them appear more greasy.
You could try giving her a sand bath - just hold her in your hands and massage chinchilla sand into her fur, she will then groom it out when you put her down and her fur will look less greasy once she has finished.
Hope this helps,
Tammy x