In the Overstated weblog (great name, by the way), Cameron Marlow suggests that blogrolls are proxies for popularity while links directly from a blog post to a permalink of another blog are proxies for influence. For example, slashdot is popular in blogrolls but Joi Ito is popular to link to in posts. Marlow does some empirical analysis showing that, while some of the very top sites show up in both permalinks and blogrolls, the two groups (i.e., the group of heavily permalinked blogs and the group of heavily blogrolled blogs) diverge dramatically in the lower ranks.
Why? Marlow doesn’t address that question in his post, but my guess is that it has to do with competitive differentiation. Bloggers who are serious about building their reputations need to find content that nobody else in their circle is talking about. If they only got their material from the hubs that everybody reads, then they would have little to make them stand out from the crowd (except, perhaps, quality of analysis and quality of writing). So competition for readership may be the reason why bloggers tend to link to sites that are not on everyones blogroll yet. (By the way, as Marlow correctly points out, this effect tends to mitigate the power law problem.
