That is quite interesting actually, but as mentioned above, when you think about all the Wii and DS games released not entirely surprising.
As an experiment I did a quick roundup of my PS3 games for comparison, although I am in the UK so have an infuriating mix of PEGI and BBFC ratings. Never-the-less, here are the findings:
18+: 15 (37%)
15+/16+: 19 (46%)
12+: 4 (10%)
PG/7+: 3 (7%)
U: 0 (0%)
Yeah, my figures are a bit different huh. In fairness if I included *all* my games across all systems they'd resemble the figures in the article a bit better. What I'd like to see is a second chart showing sales figures vs ESRB rating. Without doubt the chart would shift towards the M and T rated games.