This doesn't seem correct. I checked a few flights I have coming up and it identified a United flight as not being a MAX, when United itself tells me it is. And it identifies a Southwest flight as being a MAX when it isn't.
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.
There will be specific planes scheduled for the flight in advanced, but it changes frequently up until departure due to regular airline logistics.
(just kidding. This is a cool project. Good job)