As I understand it, the toolbar PR is derived from the "real" PR by a two step process: first the logarithm (base unknown, speculation seems to cover the range of 6 to 20) is taken, and secondly it is either truncated or rounded.
This combined process causes an immense loss of information.
Assuming rounding and a base of 10. A toolbar PR of 5 represents a real PR of anywhere between 10^5 and 10^5.999999, the latter being effective ten times higher than the former. So two sites with identical toolbar PR can have vastly different real PR.
Conversely, a site with a real PR of 10^4.999999 will have a toolbar PR of 4, while a site with the barely higher real PR of 10^5.00001 will have a toolbar PR of 5. Here the appearance of a higher PR is illusory.