Continuity mistake: When Camille asks Kryten if they could make it, Kryten puts his glass down on the table. In the next shot, it is suddenly back in his hand, being put back down again. (00:22:35)
Other mistake: When Kryten and Camille are having dinner, Camille puts her "hand" in the soup and begins slurping it up. The level of soup remains the same and doesn't lower despite being slurped. (00:22:25)
Answer: The cello is a large four-stringed instrument, which, when it is played, stands vertically on the floor between the player's legs (assuming they are seated). If it is to grow as large as the other instuments mentioned will, it would require a rather unseemly lack of femininity to be able to encompass it with the legs.
Rooster of Doom