lucy24 - 10:45 pm on Apr 6, 2013 (gmt 0)
It depends on the browser-- or rather, on the perceived age of the browser. Thanks to the "like Firefox 3.6" in its UA string, Camino is often interpreted as an old browser although it isn't really. This is sometimes useful and informative.
You get different versions of search based on UA detection. Same thing happens with image search: "old" browsers get the older version, without the lightbox business. (But with a full page load if you ask for a closer look, so you really can't win.)
"old" browsers (Camino, MSIE 5), not logged in: "Cached" and sometimes "Similar"
"new" browsers (Safari, Firefox, Chrome, Opera), ditto: no "Cached" or "Similar", but preview arrows on hover
I don't think the presence or absence of "Similar" is significant. So it's either "cached/similar" or preview but not both.
Detour to raw logs shows a dramatic change in Preview hits over the past year. At the beginning of 2012 it was nothing but Google Preview; then Bing Preview started showing up and is now about half and half. Not only because bing numbers have gone up but because google preview numbers have gone down.