Typically, at least in the past, when someone did a search and the top two results were from the same site, Google would indent the second result under the first. It works sometimes now, for example a search on rustybrick indents the 2nd result as such.

But a search on webmasterworld does not indent the result.

Same with search engine watch, and yahoo and ask.
An other thing which doesn't appear to be fully consistent. What I did notice is that the results with the Web Categories underneath them, tend to not have the indented result after them. So maybe that is the difference? See google for an example like the rustybrick example above, see no, web categories...
Forum discussion at WebmasterWorld.

Comments:
chris boggs
08/01/2006 12:21 pm
I wanna know who this rustybrick example you always use is? :P
Barry Schwartz
08/01/2006 12:26 pm
Some dork - don't pay attention to it.
Matt Cutts
08/01/2006 10:18 pm
Feature (i.e. deliberate design choice), not bug. When the #1 result has SiteLinks and there's a #2 result from the same host, the #2 result is not indented. ThomasB asked this a while ago. Umm... here: http://www.goodroi.com/2006/07/30/thomasb-notices-another-google-serps-change/
Eugeniu
08/01/2006 10:29 pm
I just clicked on link to google search and seems if you're logged to Google Account shows feeds of the referer if you're subscribed to it depending on the search query . Screenshot : http://www.dropshots.com/zoomin.php?userid=155103&cdate=20060801&cimg=0 PS : Is this english ?
Barry Schwartz
08/01/2006 10:42 pm
Matt, good eye! Eugeniu, that is the Google Coop -- see http://www.seroundtable.com/archives/003799.html
ThomasB
08/01/2006 10:51 pm
Thanks Matt for giving me credits, just to link to the original post with your comment: http://www.thomasbindl.com/blog/index.php?title=maxsuite_2times&more=1&c=1&tb=1&pb=1#comments
Matt Cutts
08/02/2006 05:04 am
Sorry 'bout that Thomas. I searched for [thomasb indent] or something like that and found goodroi's write-up about your finding first.
ThomasB
08/04/2006 12:22 am
Nevermind, let's forget about it over a delicious Sprite Zero next week. ;)