Why does Google display a 7-pack for some queries, and blended results for others?

I was testing out a search query from one of my Local Citation Finder users for 'santa rosa automotive repair' and I came across an interesting case where in one search I would get 7-pack, and then I'd search again and get blended. I've been perplexed about why Google shows a 7-pack for some queries and blended results for other queries ever since David Mihm asked about this on the 2011 Local Search Ranking Factors Survey, so I played around for a while trying to identify what might be going on.

I was able to duplicate the results multiple times. If I searched one way, I'd get the 7-pack. Make a slight variation, and I would get blended. The difference in how I searched to get the two variations was so minor, that I was surprised to get a 7-pack on one search, and blended results on the other.

Screenshots with date and time recorded:

7-Pack (click to view details)


  

Blended (click to view details)

Here's the play by play:

Search 1:

  • I put my cursor into the address/search bar in Chrome, and I type ' santa rosa automotive repair' and press enter.
  • I get a 7-pack.

Search 2:

  • I put my cursor into the address/search bar in Chrome, and press enter. Exact same query.
  • I get blended.
  • Huh?

Search 3:

  • I press the back button.
  • I type the search phrase into the address/search bar in Chrome again, and press enter.
  • I get a 7-pack.

Search 4:

  • I leave the search term in the field, and I press the search button on the results page.
  • I get blended.

I was able to duplicate these results about 10 times (I was trying to get good screenshots).

Finally, about 4 minutes later, I was no longer able to get the 7-pack. I would get blended results no matter how I searched. So, what's going on here? First of all, I think it's possible that Chrome cached my first query, and that's why searching that particular way continued to bring up the 7-pack. Searching a slightly different way somehow told the browser to run a fresh request from Google's servers. I don't know very much about how Chrome's caching works, so this is just speculation.

Assuming it was just cached, why did I see a 7-pack in one instance, and then blended results a moment later? I have a couple theories, and I'd love to hear your thoughts in the comments:

  1. Split testing. I just happened to hit it at the right time when they were just switching the test over to a new version.
  2. Query volume. Perhaps low volume queries get a 7-pack, and when I was hitting that search term, I just happened to catch it right as it passed the threshold to trigger the blended results.

I'm putting my money on split testing, because I have seen some queries that would definitely have high search volume return a 7-pack. For example, check out these two searches:
http://www.google.com/search?q=seattle+lawyer (7-pack at time of writing)
http://www.google.com/search?q=seattle+lawyers (Blended at time of writing)

Why Google decides to show a 7-pack sometimes, and blended other times is definitely still a mystery, but this is a case that seems to indicate, to me at least, that it's just split testing.

What do you think? Any other theories about what's going on?