You can thank arithmetic for at least some of the good news.

The metro Atlanta unemployment rate fell to 5.7 percent in August, the lowest rate for Atlanta since May of 2008, and elementary school math figures prominently in the good news, the state labor department said Thursday.

The jobless rate, which was 7.3 percent a year ago and 10.6 in the aftermath of the recession, came down more than expected last month, dropping from July’s 6.1 percent partly on the strength of modest, back-to-school hiring that added 4,800 jobs to the economy.

Hiring during the month was strongest in education and local government, as schools swung back into session. Hiring was solid, as well as in healthcare and construction – a of job creation in big projects like the new Braves stadium, as well as new homes throughout the region.

Layoffs too, were down, based on filings for unemployment benefits.

For sure, the overall picture is brightening. But that doesn’t mean that all of the figures in the frame are getting better.

For starters, the August rate doesn’t usually drop as far as it did last month. And it’s the magic of math that helps explain why this time the drop was double the average for the past ten Augusts even though hiring was just so-so.

To explain: the unemployment rate is a comparison of job-seekers and the total number in the workforce, so if people without work stop looking, they aren’t counted in the calculation. If they were counted, the rate would be higher.

And the labor force shrank a surprising 30,000 in August.

It’s a good bet that most of those people were not working. So their departure cuts both sides of the division problem, but proportionally cuts the numerator more.

The government says 159,000 workers in metro Atlanta are out of work and looking for a job. Add those 30,000 to that figure and the rate would be higher.

The formula for improvement over the past 12 months is parallel to what looked so good in August: The metro Atlanta economy has added a solid, if unspectacular, 66,400 jobs during the past year, while the labor force declined slightly.

In fact, despite population growth and a four-year-old job recovery, the metro labor force is only a little larger now than it was more than seven years ago.

The question, of course, is why?

A study by the Federal Reserve says about one-third of departures might be explained by retiring baby boomers. But the rest? No one really knows.

When the job market was miserable, dropping out of the search made economic sense. Going back to school, taking early retirement, staying home with kids or hanging out in the home office writing the Great American Novel – all made sense as investments of time or just avoiding a non-productive use of it when no one was hiring.

But the economy got back on its feet, the hiring has been pretty steady and labor force growth has not rebounded.

Are some jobs flying under the radar? Are the government surveys distorted in this era of disappearing land-line telephones? Are more people getting money from jobs they lie to the government about?

We just don’t know yet. And, the mystery shouldn’t be allowed to darken the portrait of an improving job market. But the question does hang there in the air, invisibly floating right above the numerator.

And one more annoying stat.

For decades, Atlanta and Georgia was among national leaders in job growth and low unemployment. Not now. As much as Atlanta’s rate has improved, it is still considerably higher than the 5.1 percent national average.

There have been 92 months since 2007. The Atlanta jobless rate has been below the national average in just one of them