You need to be more careful what reports you are reading because that just isn't true.
Just as an example:
The Presbyterian Church USA is the largest Presbyterian denomination in the U.S. PCUSA has approximately 10,038 congregations, 1,760,200 members, and 20,562 ministers. The denomination has been steadily losing members and churches since 1983, and has lost 37 percent of its membership since 1992.
On the other hand, an intirely DIFFERENT denomination .... The Presbyterian Church in America is the second largest Presbyterian denomination in the U.S. The PCA has approximately 1,808 congregations, 367,033 members, and 4,416 ministers. The denomination has been one of the faster growing denominations in the United States, growing tenfold since 1983.
I can’t say they weren’t counting startup denominations that gained from recent divisions in other denominations, but perhaps there was some establishment date they were using.
Sort of like batting average in MLB requiring so many at bats to qualify for the race.
I think you can see that if I start a denomination tomorrow with just me and have 7 at the end of the year that I’ve just experienced 700% growth.