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For a question to be automatically migrated upon close, 3 of the 5 close votes must be for the migration.
The question being discussed only had 2 "belongs on superuser.com" close votes - here's the breakdown with vote cast time:
subjective and argumentative 2009-07-26 18:58:52.980 belongs on superuser.com 2009-07-26 19:00:06.753 not programming related 2009-07-26 19:00:32.837 not programming related 2009-07-26 19:00:49.030 belongs on superuser.com 2009-07-26 19:02:20.213
Since there was no clear winner, the last close vote cast is the one used in the close reason. Perhaps a better scheme would be to choose the highest rep user's reason in ties?
Regardless, this migration threshold is in place to ensure that the community has enough consensus before displacing a question and all its answers.
From the July14th blog, Jeff writes:
As you know, there has been a good amount of talk about moving questions by low rep users to SuperUser, specifically that the site is currently in beta, and even though it is publicly available (if you know who/how to ask) it might not be obvious to the user where his/her question has gone.
With this in mind perhaps a moderator intervened, or perhaps there was a decision to not auto migrate to SuperUser for the time being.
Apologies for not providing a real answer, but adding this color via comment would have been burdensome.