The discrepancy results from the use of different age groups and different interpretations of "unemployment."
As for age groups, the U.S. Bureau of Labor Statistics breaks down monthly unemployment numbers for people 16 to 19 years old and for those 20 and older. The groupings also take into account racial demographics.
The Trump campaign takes a different view, and its economic adviser David Malpass detailed its methodology for ABC News. He said the campaign started with BLS information from all of 2015 and broke out the numbers for 16-to-24-year-old African-Americans.
He said via email that the campaign's "economic program is intended to improve the labor environment for young Americans, many of whom are getting left out," and that its methodology is more inclusive.
The campaign's figure includes not only people the Bureau of Labor Statistics classifies as labor force participants — the employed and the unemployed — but also those not in the labor force, or people who have no job and are not looking for one, such as students and retirees. The federal government does not count them as unemployed.
After dividing that larger number by the population of that age group, the Trump campaign came up with 58.5 percent.
Including people who are not in the labor force under BLS criteria dramatically affects the statistic.
For campaign purposes, Trump did not specify the age group when he mentioned the 58 percent statistic in two speeches this week and one in July. He said only that "58 percent of African-American youth are not employed."
If the same methodology is used (the same data set and the same 16-to-24 age group) but including only those the BLS counts in the labor force, that shows 9.8 percent of African-American youths were unemployed last year.
Malpass said the campaign includes those not in the BLS-defined labor force (the unemployed who are not looking for jobs) because that addresses the true state of the economy.
"Many of them would like to work if labor conditions were better," he said.
"The official labor force statistics exclude many people who would like to work if there was a better economic environment," he added. "One goal of the economic program is to strengthen the economy so that more people are attracted into the labor force, especially youths and minorities. They would benefit from the economic reforms, including faster growth, higher wages and more business investment."
But at least one economist has a different view.
"It's not proper" to include those not in the labor force in any unemployment statistics, chief U.S. economist Michael Gapen of Barclays told ABC News today.
"It's not appropriate to include people who are in school as part of the labor force and count them as unemployed," he said, citing one example. "They're consciously choosing other activities."