It is likely just changing fare bucket availability on flights. What are the fare classes involved each time you search? There are $2767 roundtrip fares are 14 day advance purchase Saturday night stay PNA46SNC P fares. If there is no longer P bucket availability on a flight in one direction or other because they zeroed it out for whatever reason, you are most likely seeing a PNA46SNC P fare in one direction and a ZNW83ERC Z fare in the other (assuming there is still Z bucket open) which prices out at around $3642 roundtrip. The only $2500 fares I see look to be partner fares. It's "dynamic" in the sense that fare bucket availability can change at any time on flights, but they don't actually load new fare filings that frequently.