Why do two of the "First Four" winners get 11 seeds in the Regionals?

I mean, if the two teams are good enough to be playing for the No. 11 seed, then why aren't they good enough to be seeded 11 and 12, to begin with? It seems to me that all four of the "First Four" games should be playing for the No. 16 seeds in the Regionals. What gives?
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