Yes, the article referenced and the last entry in the table has only 12.3% of tumors demonstrate growth beyond 2 mm.
I came across a more recent article, that was published in October 2016. It's entitled "Size as a Risk Factor for Growing Conservatively Managed Vestibular Schwannomas". It observed 555 tumors and observed that 11.9% of "Tumors that had shown more than 1-mm increase in diameter".
So, it appears that in the 1990's it was thought that about 31% of tumors grew more than 2 mm, then in the first decade of the 2000's that about 13% of tumors grew more than 2 mm and now it's down to about 11.9% that grow more than 1 mm. The trend is certainly towards reporting fewer tumors that grow. Why?
Has tumor growth pattern changed over the decades or has measurement techniques become more accurate. In the past, perhaps only using films, it would have been more difficult to take measurements. If in doubt, it would have been prudent to err on the side of caution. Now that measurements can be made using a computer, perhaps more accurate growth details can be reported.