tl;dr — A quality mark of a dissertation is that its reviewers don’t all come from the same university. However, different universities have different grading systems, making average grades meaningless at best and hurtful to the careers of young researchers at worst. So we should move to a pass/fail system of grading.
A dissertation (a.k.a. Ph.D. thesis) usually has at least two reviewers. The first one is the main supervisor at the doctoral student’s home institution. For a quality dissertation, it is smart to have a reviewer from a different institution but the home institution, to avoid the smell of inbreeding or impropriety. Some universities even require this.
However, different universities have different grading procedures and calibrations. What is average can vary significantly across different universities. Even when instructed as to what the average grade at the home institution is, most colleagues in my experience will find it hard to bend to that and will still be influenced by their own institution’s grading. Then, unless the other institution’s grade inflation is higher than the home institution’s, the doctoral student will be disadvantaged when it comes to the final (combined) grade of the reviews. This lower grade when compared with the local competition will make them miss out on the usual litany of local (and mostly irrelevant) prizes or commendations that are still important for a career in science.
As a consequence, we should do away with absolute grading and only grade in relative terms on a curve to ensure fairness for quality dissertations. Since this is unlikely to happen, in my opinion, we should just drop grading altogether and leave prizes and the like to external committees and evaluations.