Drearyweather ...
It just shows a trend or a direction year on year. Just taking kids becoming publishers and then baptised. Imagine a funnel like you might use to pour oil into your car engine.
At the top are all the kids who have a parent as a jw. When they reach an age between 6 and 14 (eg.) some will become publishers. When some of those reach an age between 12 and 18 (eg) they will get baptised.
So the more publishers you stuff in at the top then the more baptised dribble out of the bottom. The higher the increase in the worldwide publisher number then the more likely that the baptism number will increase in the following years.
Pressuring to lower the age of the kids getting baptised also gives an increase. But then it cannot improve much further. You might bring the average down from say age 15 to age 13 but any younger and it gets more and more difficult to justify and people resist. Especially in western lands.