Why does Kayak's "x seats remain" number go up and down, and what does it mean when it goes down to 1?

There’s a flight I don’t want to miss buying (unusually cheap, perfect for my intl trip) and one of its 4 segments is down to “1 seats remain.” But I noticed that this segment was “5 seats remain” and THEN “9+ seats remain” yesterday! Should I take the 1-seat warning seriously and go ahead and buy the ticket? Ideal would be waiting a few days so my bank acct doesn’t go too low, but I have enough for the ticket if necessary.