I'm not sure this has anything to do with your private browser windows, etc. because I don't know all your details. But in practice there are always limited seats available in the lower fare buckets and they are released (effectively from the higher buckets but that concept may be meaningless really) in dribs and drabs. So it does pay to look far enough in advance that you are not in a hurry to buy, and keep on checking. Seats seem to get released overnight and I see some correlation with weekends, but that may just be because travel agents aren't up early working them as well.
Anyway, good job on getting a good (I hope!) fare.