Memorial attendances are pretty volatile. They are not the best statistic for evaluating growth anyway. In my view the best statistic in that regard is the number of congregations, because it's a figure that reflects long term trends rather than contingencies such as the day a meeting falls on. Plus it is a figure that can't easily be faked or manipulated. Individual publishers may put in false reports, or the organisation might relax the requirements for pioneers, thus skewing those figures. But a congregation is a congregation. It is a legal and social entity that has a secular as well as an internal imprint and arises only through local effort independent of fudged figures or organisational requirements.
Congregation numbers is one of the key reasons we know Mormon growth figures are bogus for example. For years they have been claiming growth in various places, while simultaneously the number of congregations has been going down. How come? Because their membership figures are grossly inflated by not subtracting those who fall away from the religion from their numbers. But the congregation statistics don't lie, Mormons are in significant decline in most of the world, including poorer countries.
As you correctly say, I don't believe JWs have the truth, and if they genuinely started to decline I would be all over that! But it hasn't happened yet. In fact predictions of JW decline on this site have come to resemble JW reports on the nearness of Armageddon - "just around the corner", "all the signs are in place now", "maybe next year".