You cannot use number of baptisms to calculate increase, because publishers are exactly that - publishers - before they are baptised. Baptism candidates come from the ranks of publishers, and added to that, some start as publishers and are baptised within the same service year, whereas others "cross the year border".
Additionally, nobody when doing these calculations seem to bother about the fact that around 1 % of a population die during a year. Hence, a 0 % growth "in reality" is a 1 % growth because the 1 % that have died have been substituted. a 3 % growth therefore "in reality" is a 4 % growth.