The numbers in parentheses are the additional decimal points which have been measured but lie outside the accuracy of the measurement process. This means the values in the parentheses eg (15) could be used for the purpose of calculation, but any value that results from such a calculation would not be more accurate than the number of significant figures if those in parentheses were not considered. In other words, the values in parentheses are there to help avoid rounding errors if you’re trying to work to the limit of accuracy of the gas constant. I agree that this is not commonly known, and only confuses the issue. I would recommend removing it.