The Last Day - S3-E6
Revealing mistake: You can tell Robert Llewellyn is reading his lines when Kryten receives the computer chip. He is looking much lower than the chip in his hand and it was obviously quite a complicated piece of dialogue to remember. (00:12:35)
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