Just as 9S suspects, Rita has pretty much never cleaned the place. Childermass was certainly the cleaner of the two of them, but between their month-long absence while in Portland, the time he took to recover from injuries, and then his sudden departure, some tasks were put on hold for a while. And when those tasks fell to Rita... she couldn't be bothered.

"Hey, it's not that..." Rita begins to protest 9S, but Pod 153 interjects before she can finish the sentence. "--You too!? Come on, it's just a little dust. If it bothers you that much, then you clean it." She folds her arms and humphs at them. As far as she's concerned, this is a pretty normal level of cleanliness. What are they, a couple of neat freaks? Maybe androids are used to more sterile environments.