When maxdig is larger than 8 and the number is smaller than 0.00001, the number is rounded to 8 numbers after the coma. When the number ist smaller than 0.0001 the maximum numbers after the coma is 7, and so on.
References
C. Reimann, P. Filzmoser, R.G. Garrett, and R. Dutter: Statistical Data Analysis
Explained. Applied Environmental Statistics with R. John Wiley and Sons, Chichester, 2008.