I have been educated in statistical methods in my scientific background. I find the above comments grounded as to possible explanations for the increase. Here are some summaries and other thoughts.
1. It seems these statistics are based on the publications of the religions in general. The religions themselves may have non traditional methods of statistical analysis.
2. It is generally accepted by statisticians of religion that because of church pressure and fear of shame self reported religious statistics are the most accurate indicator of religious beiliefs not merely cultural obligation and birth rates. In other words, individuals have to say in private surveys "I am a JW." Many members of all religions are "members" but not "believers." This confuses the issue of growth of a population vs. growth of an idea.
Traditional problems for researchers studying Jehovah's Witnesses include
1. Refusal to participate in secular research studies.
2. Ambiguity on the terms of membership as defined by the sect.
I think the most accurate statistic to answer the question of "Are Jehovah's Witnesses growing in influence in America?" could be best answered by differentiating between converts and those born into the religion, dealing with retention rates and conversion rates. Jehovah's Witnesses will not submit to such a study, so only a self report could get at this information.
Interestingly, the increase can be largely explained by the mentioned change in time keeping policy coupled with the dynamics of increased pressure to report volunteer time. This street runs both ways, however, because if the same standards of attendance and self reporting were applied across the board there would be many more statistical Jehovah's Witnesses. In other words the statistics published by the WTBTS only represent the actions and activities of the most dedicated members of the group. So the JW faulty statistics are only an indicator not a comprehensive measure. It's like basing the measures for global warming only on the population of polar bears. It is clear to me that the Jehovah's witnesses statistics department noticed a decline in "peak average publishers" or "average peak publishers" and has interpreted this to mean that "the times are harder." Thus the proceeding decrease in standards for publishers. I estimate there are probably 20% inactive JW's who would still self report as JW's in survey. So JW's have a long time in which they can gradually slack the standards of membership and report an increase that is really just a function of the standard of selection.
Jehovah's Witnesses can often be viewed in terms of business metrics since their major work is a form of religious "marketing." They have adapted well to changing market niches. The basic problem is that they are declining in their original market niche and the organization is culturally rooted to middle aged / elderly american whites. What will happen is that as the traditional niche becomes marginalized, and say only a minority of JW's are suburban white americans, there will be a cultural need experienced by majority populations to see their unique cultures more represented. I predict there will be increasing numbers of foreign JW's finding themselves to be "anointed." I predict a statement in the next several years along the lines of "And doesn't this make sense, brother's? Jehovah is not partial to any race or ethnic group. He has mercifully extended the end times in order to see his heavenly slave class include people of all nations, as evidenced by the "Twelve Tribes" of Revalation meaning a balance of anointed ones from all "tribes" of the earth!"
Think of Mc Donalds changing their slogan to "I'm loving' it."
I have agreements from many of my friends that the moment such a statement is published they will read everything I have to say against JW's.