The Super Bowl is the most-watched show on TV, year in and year out. Nothing else can even come close. And if you look at the game's viewership trends, it's a pretty safe bet that each year's Super Bowl will become the new most-watched program in American TV history.

Yet at the same time, the Super Bowl's ratings have remained more or less consistent since the 1980s. Indeed, the highest-rated Super Bowl to date aired in 1982. Even as more and more people watch the big game, the percentage of Americans who tune in has stayed at roughly 45 percent.

This is all thanks to how Nielsen calculates its ratings, the growth of the US population, and the slow decline of just about all other television programming. Or, put another way, it might seem like everybody in the country watches the Super Bowl — but only a little under half of us do.

Why viewership growth doesn't reflect ratings growth

A ratings point — calculated to the 10th decimal place — is a percentage of the number of American households that own at least one television and were tuned to a particular program at a particular time.

It represents 1 percent of the roughly 115 million American homes that own a television set, or 1.15 million homes. From there, Nielsen estimates viewership based on its demographic samples, which account for age, race, family size, and other things.

One of Nielsen's "homes" could be a family of five, all watching the big game, or it could be a single person, or it could be a big Super Bowl party. The company can't ever know with 100 percent certainty how many people are watching the game in any of these homes (that single person might invite a friend over!), but it has a pretty good guess.

Thus, Nielsen's metrics are, to some degree, imprecise, but the company's statistical sample is a fairly accurate rendering of the country's demographics in miniature, so its viewership estimates are about as accurate as such a statistical method is likely to produce.

When you look at the graphic above, you'll see that the overall viewership numbers for the game have climbed, more or less nonstop, for the past 30 years. And yet the rating (again, the percentage of people watching the game) has remained fairly stable. For example, the highest-rated game of the past 30 years was in 1986 — when approximately 48.3 percent of American TV viewers watched the Chicago Bears destroy the New England Patriots.

The reason, of course, is that the American population keeps growing. Forty-eight percent of Americans in 2015 is a lot more than 48 percent of Americans in 1986, which explains how there can be a gap of more than 20 million viewers between those two Super Bowls (92.6 million in 1986 versus 114.4 million in 2015) even as the 1986 game was two-tenths of a ratings point ahead of the 2015 game.

It also explains how the M.A.S.H. finale, which aired in 1983, remains the highest-rated TV show in American history, with more than 60 percent of the country tuning in, even if it's no longer the most-watched.

The reason the Super Bowl is so much more valuable now, then, isn't that its ratings growth has been so explosive. It's that it's the only event that reliably delivers such a big audience.

Awards shows like the Grammys and Oscars do okay, but they rarely top the 45 million viewer mark and certainly don't hit the same ratings heights (again, because a smaller percentage of Americans watch those awards shows than the percentage that watch the Super Bowl). The World Series used to be a big draw but is now a much more modest one. And outside of heavily touted series finales (which also aren't what they used to be — see Lost in the graphic above), regular episodes of most shows can't even compete.

One final note: The network that airs the Super Bowl in any given year will often brag about "total viewership." In 2015, that number was 168 million. But "total viewership" simply measures the number of people who watched the game for at least a tiny portion of it, as opposed to the whole way through. While total viewership is interesting to know, it's not as reflective of the actual number of more dedicated viewers who watched the entire game.