How do you know this isn't a case of someone else buying a ticket while you are thinking (selling out a fare bucket, so price goes up), and then someone changes flights or RM just decides to release more space a few minutes later, so that fare bucket opens up again just around the the time you are deleting your cookies.

Next time you see this, maybe open an incognito window (to see low price) and then refresh in your normal browser (to see that the price is still high)?

Otherwise I'm inclined to believe that the price just went up briefly and then came back down and the timing was coincidental.