Niche systems can sometimes get a higher attach rate due to the fact that a larger percentage of their userbase is the heavy hardcore buyer. As far as I can tell, of the mainstream systems from the last few generations, it was GameCube with the highest tie ratio in Japan after one year.
Of course, in this case the fact that X360 has been out a year more than PS3 or Wii doesn't hurt, either.
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Of course, in this case the fact that X360 has been out a year more than PS3 or Wii doesn't hurt, either.