I have been noticing this with a few of my top keywords. For instance, my top keyword produced AT LEAST 1.2 million results. It's now only showing up 535,000 results. I have noticed this with a lot of keywords, and it looks like Google is cleaning up house! I dont know if the other sites got banned, but they dont appear to be in the top. The new top 10 are sites that were on page 2-3, and mine is gone!
It may or may not last, I had one term several weeks ago. It said 270K results for weeks, then one day it was at 6 million. I like your scenario better
They are cleaning and doing a lot of changes. Even our website is already at the top 6 and when i checked 3 days ago it was already in the top 400 only for that certain keywords.......