There are a bunch of different class fares for all the coach seats on the plane. Some priced high, some low. When someone buys one or two the next available tickets go on the block, most likely more expensive.
When I bought my 2 tickets last week for $521 the price jumped to $581 when I searched for the same flights again after my purchase.
The browser cache thing is a myth as far as I know. You can always use
http://matrix.itasoftware.com/ which is great at searching all options out there.
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