Add a cello to a loadsheet? You aren't serious? Why, it weighs less than many other carry on bags? What is it that's so special about cellos that they seem to take on quasi-human form when in an aircraft? Blankets and bags in empty seats aren't counted, why should a cello be different?
Because a boarding pass will be issued to the cello, since it's occupying a seat, and issuing it a BP is the easiest way to ensure that no one sits in the place reserved for the cello, since you won't have 2 BPs for seat 23E, or whatever. You then make note of the cello, so that you know not to use a standard adult passenger weight, but to use some lower number (I actually have no idea how much a cello weighs - 15kg, maybe, but certainly not the 77kg that you'd otherwise use).