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But what is clear in watching our own sites over time and hearing other members comment is that the share coming from each SE can vary significantly by category. The nature of users at the three major SE's is not exactly identical and that plays a role.
Also, there may be things about any given site that cause it to rank well with one SE and not another. In some cases for example we have more trouble ranking well in Y than G. In other cases, the opposite.
There are a lot of Webmasters that build some of their sites with one SE in mind. We don't do that, preferring to make sites that hopefully rank well in all SE's (by virtue of what each site offers). But it doesn't always work out that way.