iPhoto - How are Events segmented

[uptownnyc]

I'm trying to understand how iPhone clusters its "Events". If I look at the "Events" option under Library, I see them clustered as monthly groups (i.e. "December 2012 Photostream", "November 2012 Photostream") but I also see some one-off photos that appear as stand-alone events ... or sometimes it's a bunch of photos (but not an entire-month's worth).

Any ideas how this happens, or what causes a single photo to appear as its own event? Is it based on GPS location, or something else?