Re-reading the Google blog post, I'm struck by these terms:
In addition, this change also goes deeper into the “long tail” of low-quality websites
and
where the algorithm might not have been able to make an assessment before.
I think these two phrases are very important:
1. Why would the initial algorithm not impact all sites and all pages in the index? why would they roll out it phases?
2. Why would the algorithm not be able to make an assessment before, but, be able to now?