I was still talking about the last 50 games, in which case it is most definitely true.
I made a new, up-to-date Excel sheet with all GameSpot scores and some totals:
http://www.niwra.nl/tmp/DSvsPSPGamespot.htm
I will try to see if I can collect data from different sites in a similar way. Right now, gamerankings.com seems offline, but I've put some work in getting all data from IGN in there too.
It is true that IGN seems to have more balanced figures. For year-to-date, the PSP still has the edge, but only slightly. It has 12 more games (60 vs 48), more 8+ games (14 vs 11), and the same number of 9+ games (3 vs 3). Also, the average ranking of these games is over a point higher (7.11 vs 6.07).
In the overall totals, with 126 (PSP) vs 132 (DS) games ranked, the PSP games still score a higher average ranking (7.23 vs 6.43), but the DS has more 8+ games (37 vs 32) and more 9+ games (10 vs 6).
Taking these two sites together, the two systems seem to be fairly equal in terms of over 8+ scores. If we assume your Gamerankings findings are more reliable than the average of these two, then it seems that the PSP is behind in the 80% category overall , but still, as you say, not by a large margin. And even then, the trend for 2006 seems to be that the PSP may be catching up and is certainly doing well so far this year.
Of course, that leaves in the middle whether scores on different platforms can actually be compared at all. Is a game that gets an 8 on the DS comparable to a game on the PSP? Ah, questions, questions.
But it's still interesting to ponder over. Maybe it is also interesting to do some kind of 'best of category' (cf Ace Magazine's A-list, which was great) for each platform and compare the games again there.