Editor’s note: The initial headline on this piece was, “How is it that Bernie Sanders still averages $27 per donation?” As Philip notes in his thoughtful recap of the criticism it has received, it was changed upon publication to a more provocative version. As described below, Sanders’s average donation has indeed fluctuated above and below $27, but the piece was written to be an explanation of the oft-repeated claim rather than a debunking of it.

By the time Larry David appeared on "Saturday Night Live" to spoof Bernie Sanders in early February, his talking point about his campaign's average donation being $27 was already well known enough to be a punchline.

That was two-and-a-half months -- and a whole lot of individual contributions -- ago. And yet, Sanders still regularly mentions the same stat: His average donation is $27. Here he is in Arizona at the end of last month.

Democratic presidential candidate Bernie Sanders asked, "Anybody here know what the average contribution?" in Phoenix on March 19. The crowd shouted back "$27!" (Bernie 2016)

He even mentioned it at Thursday's Democratic presidential debate. Which prompts a natural question: How? How is it that the average has stayed so consistent over time?

A few ideas:

1. Averages change more slowly as you add more data

A quick math lesson: You calculate an average by taking a group of numbers, adding them up, and dividing by how many numbers you had. So the average of 9, 10 and 11 is 10 -- 9+10+11 equals 30, and 30 divided by the number of numbers (three) is 10.

What that means, though, is that as the number of items tossed into the average grows, the average itself changes more slowly. After all, the number nine in the example above is one-third of the numbers included in the average. Sanders's campaign has received north of 6 million individual donations. So any one donation is only 0.00002 percent of the total -- and therefore has a much smaller effect. There's a lot more variability with smaller sets of numbers than there is with larger ones.

If we graph the reported contributions to Sanders's campaign -- data that excludes a lot of small donors who are below the limit to need to be reported -- you can see how much more slowly the figure shifts as more contributions are made. You can also see how the average has dropped over time, thanks to the surge in small donors the campaign has seen. (Again, the average here isn't $27 because smaller contributions aren't itemized, so we can't crunch those numbers.)

2. It became a thing

What's more, the $27 average has become a symbolic rallying cry for the campaign -- a demonstration of his reliance on small donors. That speech in Arizona makes it clear: People know what the answer to his question is.

That's reflected in the FEC reports, too. The number of reported $27 donations grew quickly in February.

Which suggests that giving $27 is a point of pride for Sanders donors.

3. His website encourages that number

The campaign reinforces that idea on its website. Here are the suggested contribution levels.

It's a nice number for the campaign, bite-sized. If you were going to give the guy $15, as suggested, bumping it up to $27 isn't really that big a difference. And what's more, it's the sort of thing you can do again the next time you have an urge to give.

But the real answer to the question is this:

4. $27 isn't really accurate

At its heart, the idea is just a talking point. Consider the campaign's press statement after the February reporting period.

"The Sanders campaign in total has tallied more than 4.7 million contributions, compared to [Hillary] Clinton’s 1.5 million," it concludes. "February’s fundraising brings the campaign’s total raised this cycle to more than $137 million."

$137 million divided by 4.7 million is ... $29.14.

More than 4.7 million contributions means, at most, 4,749,999 -- or else the campaign would round up to 4.8 million. Even with that higher number of donations, the average is $28.95. Which is more than $27.

In March, the campaign was apparently under that mark. Its real-time donations tool indicates that $44 million was raised from 1.7 million contributions -- about $25 on average. Combining the total through February with those figures, the average drops to $27.88 -- or $28 on average.

All of the factors above are still true. As more donations come in, the average will still be in the same ballpark. The campaign encourages those $27 donations, and his fans are eager to oblige.

But is the average $27 every day? Not according to data from the campaign.

Update: Bernie Sanders's rapid response director, disagreed with the article on Twitter.

So I asked for specific numbers. And I got them.

My estimate was one cent too low. I regret the error.