Planes get changed, especially if the flight is far out. If not enough people buy a ticket for a 737-8 sized plane, SWA will sometimes swap it out with a 737-7

Planes get changed, especially if the flight is far out. If not enough people buy a ticket for a 737-8 sized plane, SWA will sometimes swap it out with a 737-700 with a smaller capacity. This also allows SWA to fly with one less flight attendant than they would on the larger plane. If there is a maintenance issue with your 737-Max8, and there is a 737-800 sitting in the next gate not set to go out until a few hours after your plane, they'll swap it out to reduce the chance of having a late departure. If the site is not scraping data every minute, there could be mismatches. It's unlikely this site is scraping data more than once a day to save costs.