Okay, I'm a little confused here, Grumpus. Do you have a source for any of this you can share? My understanding of PR is significantly different from this description.
Actual Page Rank is calculated on a curve only in the sense that everything statistical can be graphed on a curve. All pages on the Internet start out with the same exact PR, generally thought to be equal to unity. As "votes" are cast, those pages with the most and best links will have their PR increased. If every page on the Internet linked to this page and this page only, the PR of this page would be about three billion multiplied by the dampening factor. All the other page would remain at unity. Yea, that's a curve, but it certainly isn't a bell curve. Of course, in reality, we probably get much closer to something resembling normal distribution, but it's nonetheless NOT based on averages or means.
The GTB is simply a representation of actual PR, designed to help us "see" actual PR, and it's curve is generally thought to be pseudo-logarithmic. We don't really know what the PR is of a site with a GTB rank of 5, we only know that it corresponds to other sites with a GTB rank of 5. Go to the Google Directory and look at the Page Ranks depicted there. They, too, are only a representation of actual PR, and they do NOT correspond exactly to the Tool Bar scale (instead of going from 0 to 10, the Directory PR runs from 0 to 7). A logarithmic scale only tells us the hill is going to grow increasingly steeper, making it much more difficult to go from 4 to 5 than it was to go from 3 to 4 (many think four times harder, but no one outside Google really knows the scale). To suggest that "the average site on the web is PR5" essentially means there will be more PR5 sites than PR4 sites and that simply contradicts the math.
It is possible, and maybe even probable, that the number of pages in Google's index will affect the GTB because it is, after all, a scale with a beginning and an end. That does NOT, however, mean that the number of pages in the index will affect actual PR. Nothing about Page Rank depends on averages or means.
If we think of Google's PR as a voting system, which was the way it was designed, it becomes more clear, I suspect. No matter how many people who turn out to vote next November, we will NOT rank the Presidential candidates on a curve. Fortunately for the American public, this analogy quickly breaks down because ABC, CBS, et al, haven't yet come up with a Tool Bar to help us "see" the election results.
So, like, what do you know, Grumpus, that I'm apparently missing? Please tell me your post was just the result of not enough coffee this morning, 'cause this confusion you're causing me is very uncomfortable.