It might seem like they could get away with that, but they really can't. It would work for a year or two, but it would quickly become very obvious due to compounding, the effect of percentage increases over many years.
For example, if a country had 500k publishers and they reported growth of ten percent a year, they would have 978k publishers in only seven years, almost double. People in that country would probably be aware that no congregation they know of doubled in the last seven years. They would also wonder why they didn't need more kingdom halls, more magazines, books, etc. It also wouldn't match data from people like the Pew research center. I think most people in the organization are basically honest, and something like this would raise concerns among too many people.
It's like companies that fudge the numbers to prop up stock prices. Creative accounting can only get you so far. This actually happened in a company I worked for. They lied, hoping the market would go back up, but it didn't, they got caught out and people went to jail. I think they would do this if they could, but it's just not something they can get away with.