The following shows link how to map a PD to a S&P rating:
|S&P Rating||PD range [%]|
I know that this mapping only is what the writer of this paper have come up with, but it shows a general trend I have observed by working with credit risk for many years.
Bad risk grades do most often have wider PD ranges than better risk grades. In this table the PD range in B is from 5.44 % to 14.21 % but it for A is 0.09 % to 0.23 %.
Is there a mathematical/statistical reason for that, or it is only practical. With practical I mean that there probably is not so big difference between a PD of 7 or 10 % when you are going to invest in a corporate.