Google has been making changes to its local search algorithm (the formula that determines rankings).
Google is, among other things, penalising category spam.
Here is a good article on that:
Google’s Recent Changes to its Local Algorithm
As of October 2009, Google has also changed from the 10-pack to a “lucky” 7-pack.
Here is another article about Google’s changing local search strategies. This one is not about the most recent changes but gives an idea about the future.