Adjusted for inflation numbers are fun to look at, but shouldn't be taken seriously because those films had less competition. Superman: The Movie came out during a time when fewer films were hitting theaters than today, there were no services like Netflix grabbing peoples' entertainment dollars, no video game consoles to eat into the pie, etc. There are dozens of factors to include when trying to use inflation numbers (though most will ignore those factors), and using inflation alone gives a false sense of how the film stacks up compared to todays films. Superman: The Movie sold a lot of tickets, but also had way less competition.