The X360 has a decent bit of very very good games that have sold to over 10% of the userbase in Japan (DOA4, RR6, N3, BD, DR, DOAX2, GOW, IdolM@ster, TB, Oblivion). This is in stark contrast to the Wii which has Wii Sports, Wii Play, WW:SM, Zelda:TP, DQS and MP8. That's 10 to 6. That is why the ratios are so different.
With DQS and MP8 out, the tier ratio has increased quite a bit. It was 2.1 to 2.2 just before DQS, so the ratios have improved a strong amount. But still, 2.5 on the leading next-gen console isn't ultra-great. Not if the Wii was to land total domination against the other next-gen competitors.