Spreadsheet comparison of all the boats which got a lower D-PN number 2003 compared to 2002 shows that D-PN number change % is much greater than the % change for a given wind range.

Specifically, the GCat 5.7 numbers for the last 4 years from the US Sailing site :

G-Cat 5.7m G5.7 72.0 78.0 74.3 72.0 66.2 2003
G-Cat 5.7m G5.7 75.1 78.0 77.2 73.0 68.7 2002
G-Cat 5.7m G5.7 74.7 [ 78.0] 77.2 ( 73.0) [ 68.7] 2001
G-Cat 5.7m G5.7 73.8 [78.0] 76.4 (73.0) [68.7] 2000
I would say that the D-PN number for the GCat5.7 had crept upwards for some reason, and has now been corrected back to its original range. I don't know why the committee changed the number, maybe someone who raced GCat5.7 in the last few years suddenly got significantly better, and this performance based system is reflecting that.

When I use to race a NACRA 5.2 in 1988-1992, it rated 72, and we used to race against a GCat5.7, which also rated 72 at the time. Both boats were about the same speed around many diferent courses. Current Portsmouth numbers for both boats are the same, again.