Actually the JW numbers published are generally lower than most other religions.
The JW Publisher numbers do include a large number of non-baptized ones, mainly young ones whose parents are JWs. Once a young one "participates" in the preaching work, they are, by default, publishers and can count time. Just to throw a number at the wall, the age would be somewhere around 5 years old or so, depending on the child and the BOE.
On the other hand, many people claim to belong to a particular religion and have little if anything to do with it and the numbers are greatly inflated. Think about how many people claim to be Catholic but may never have set foot in their church since they were a child ... if at all. The same goes with many other religions.
Rub a Dub