The poster lives in SFO and wants to visit XYZ for a week once a year. Rather than buy SFO->XYZ->SFO tickets (which are packages that consist of “a flight from SFO to XYZ on the 1st of April” and “a flight from XYZ to SFO on the 8th of April”), the poster instead buys XYZ->SFO->XYZ tickets (which are packages that consist of “a flight from XYZ to SFO on the 8th of April” and “a flight from SFO to XYZ on the 1st of April next year”).

Because the poster is repeating this purchase every year, it results in the same long-term flight pattern. However, because they have shifted the airline’s window into those travel patterns by a half step, all of the flights look cheaper to the airline’s pricing algorithm.