To fudge the numbers would require collusion among several high-ranking JWs as follows:
Each country tallies its own set of numbers based on what individual congregations submit (e.g., monthly publishers, Bible Studies and annual baptisms,Memorial attendance, partakers, etc).
Those numbers are then forwarded to the Bethel office and/orJW organization headquarters. So individual JWs assigned to forward those numbers would know if those numbers were later fudged for publication.
So, yes, fudging the numbers is possible; however, it would require the willingness of several JWs in higher positions to collude with the fudging - and the more JWs involved in this process, the greater the likelihood of leaks and disclosures about the fudging.