I love it when this question pops up, as it often does here. First of all, the premise of the question is that the Society's growth is currently positive. But that assumes the WTS' numbers are accurate, and it's long been established that many of their numbers that we know something about are inaccurate. Example: The billions of hours of field service that show up on their reports each year are totally bogus, as everyone knows... the numbers are fudged, made up, or estimated by countless dubs (and dub congregation secretaries).
The Society knows this and shrugs it off. So why would any of their other numbers be right on?
The truth is that, in my little corner of the world - California - there is no doubt the growth has gone negative. And California is a bellwether, often predicting trends that will soon be nation- and world-wide. In an area where the population has doubled in the past 15 years, the number of congregations is unchanged, despite some artificial growth engineered by the Circuit Overseer some years back when he insisted the local dubs buy a lot and built two new KH's on it, to accommodate "all the future growth." they did, and what happened is that two congos that were each sharing halls with two other congos moved into the new buildings. The number of congos did not change. The number of publishers is pretty much stagnant (in fact, in the late 90's and early 00's, when I was active, the attendance was dropping dramatically, despite massive move-ins of new citizens from metro areas, including quite a few dubs).
In our old hall, in another state, we received word recently that the two congos that share a hall built in the early 1990's are still sharing the hall, still have about the same number of publishers, and there are no new congo's anwhere in the area. This despite that fact that the area has exploded with population, more than doubling in that time.
If you contrast that with the 15 or so years prior to 1990, there were new congos formed and new halls built everytime you turned around, especially in high growth areas like the ones I have described. There can only be one conclusion from these observations, and that is that dubs are losing people out the back door as fast (or faster) than they are coming in the front. I think this is a trend that will soon be obvious throughout the country and the world.
I don't believe for a minute that their "growth" is in positive territory, in real terms. To me, the demise of the WTS, spurred on by dubs voting with their feet, started several years ago.