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<p>This is total nonsense. “Significant digits” refers to values that are known approximately up to some accuracy. In the GPA calculation everything is known exactly, because the numerical conversions are simply <em>defined</em> to equal particular values. Which values are chosen might vary from school to school, but that has nothing to do with significant digits. </p>
<p>Now, you can view grades as being imperfectly known in a statistical sense. That is, there are some random ups and downs in the assignment of the grades. So in some statistical sense, an A-minus is 3.7 plus or minus 0.1 (or whatever level of inaccuracy you view it as). Because of the averaging over several dozen courses, the “GPA variance” will be about an order of magnitude smaller than the “individual course-grade variance”, and in that sense, the GPA is accurate to an extra digit or so beyond the accuracy of the individual course grades. Not two digits more accurate, but one digit is a safe assumption, and it’s defensible to report the second additional digit as well.</p>