The airlines' Yield Management systems work on a bid/offer basis, when a request for a seat comes in they look at who else might bid for the same seat(s) and decide on the potential value and decide whether to accept the offer, reject it, or push it up into a higher yield management 'bucket'. It's very complex and hard to explain in a few words. They can also identify the point of enquiry and bias the display according to that.
For this and other reasons it is highly likely that the price of the same seat on the same flight can fluctuate depending on who is asking for it and when. Frustrating, but as MathFox said, you can gamble, you might win, you will more likely lose (which means the airline wins!).
Strangely for an industry losing money, out of over 40 flights that I've done this year, very few have not hit at least an 80% load factor, and in most cases, 100%.
You can sell anything if you sell it cheap enough. A full flight does not necessarily mean a profitable flight, unless you're Mr. O'Leary!