Is not that they lie (which they do).
If one day your page has no PR and 2 days after it does it's simply a glitch; sometimes even refreshing the page "solves" it.
But pages which we keep the n/a value for longer time... sometimes this is not a bad signal, it's simply that they do not have links enough. I have seen pages with good PR not ranking, and also even seen some correlation to traffic/interest: I have a review site with a very good arquitecture, almost every page has the same importance, but pages with no reviews rank lower (and have less PR). And it makes sense: they're of less interest to visitors.
Let's not worry about PR, but let's do about having a useful page that has everything the others do but all together in one place.
Try it: have a good site with good arquitecture and you'll see the most useful pages will rank better and have better PR, even with the same links.
As of now the PRTB (pagerank that you see in the tool bar) means less than never.