For the animals on farm, the Cool Farm Tool works on a whole year basis and so we are trying to calculate on average how much manure and methane is produced by these animals in a year. If the animals are only present for half the year, you only want to calculate half the emissions. Therefore, only half the number of animals should be entered.
For the number of animals sold, we are interested in the total weight of meat that is being produced in order to allocate emissions against that volume. Therefore, we need the total number of animals, rather than the average.