Q: Why do some photos float between events?

Sorry, I should have been more clear. I'm in Photos view, and all my photos are in groups (with a little triangle on the left that can expand them); these groups are named by the names of the folder which was dragged to iPhoto to import them. I thought these were the events. My last import, oddly, stuck a bunch of photos inbetween two such groups - they don't belong to a named group somehow. Any idea what this is?

Oh you mean each photo got brought in as its own event, and when that happens, there's no event name (with expansion triangle) shown?