This is a recent personal experience of mine. In early May I was looking for a BOS-SFO flight in August and found several $599 fares around my desired date. While I took a day or two to confirm my plans in San Francisco, by the time I was ready to book the flight, all the $599 fares are gone. I could only see $899 and $1,199 fares not just around my date but up until the end of schedule. Since then, I went back to re-check prices once a day and noticed that the $599 and $549 fares were being offered only in the time window of 3 days to a month to departure. So I waited. And finally the price which went from $599 when I initially found it to $1,199 came back to $599 and I booked it then.