One of the most basic maxims in labor relations is that union members earn more than nonunion members. And, for the most part, the numbers bear that out year after year. In 2012, union members earned a mean hourly wage of $25.71, while nonunion workers made $21.27, according to the just-released 2013 edition of our Union Membership and Earnings Data Book. But within those figures are some interesting exceptions and trends that are worth examining more closely.

For example, even though union members earned about $1.21 for every dollar earned by nonunion workers in 2012, their mean hourly wage fell for the second year in a row—only the fourth back-to-back decline in the past 30 years. Nonunion workers, on the other hand, saw their wages in 2012 grow for the first time in four years, narrowing the gap between union and nonunion pay to its third-lowest point (79.1 percent) since the Data Book started keeping track in 1973. (Note: All of these figures are in 2012 dollars to account for inflation.)

Also, for the first time since 2008, and for only the second time since 2003, the union-nonunion pay gap for public-sector workers ($1.18 in union wages for every dollar in nonunion wages) was larger than the gap in the private sector ($1.17). Historically, private-industry pay planners have favored union members to a far greater degree than their public-sector counterparts have. But while the pay gap has steadily fallen in the private sector—the ratio used to be as high as $1.35 in the 1980s—it has stayed relatively the same in the public sector.

Finally, there’s quite a variation among industries when it comes to the pay gap—and in some, the gap has become upended altogether. I’m looking at you, manufacturing. In 2000, nonunion workers’ pay finally caught up to union members’ in factories, and it’s been that way ever since. In 2012, union members earned just 96 cents for every dollar earned by nonunion workers. (It’s worth mentioning that, when benefits are taken into account, union workers still typically earn more in terms of total compensation.) The other industry group that shows union wages lagging of late is wholesale and retail trade, where the ratio has dwindled from as high as $1.44 in 1981 to 94 cents in 2012.

Industries in which union members still earned more than a dollar for every dollar earned by their counterparts in 2012 include construction ($1.46) and services ($1.25).