In all those countries on the left, the incomes of rich households rose at faster rate than the incomes of poor households.
Why is this happening? Here are three possible answers from the report.
1. Robots, etc.
Trade barriers have come down. Technology has advanced. The combination of these two factors has disproportionately benefited highly-skilled workers. You want to be the guy building the robot, not the guy whose job got replaced by a robot.
2. Rich people marry rich people
Inequality is calculated by household, not by individual. And a few changes at the household level have driven some of the increase in inequality.
For one thing, it's become more common for people to choose spouses in their own income bracket. In other words, rich people are now more likely to marry other rich people, and poor people are more likely to marry other poor people. (There's a creepy term for this: "assortative mating.")
Single-parent households and single-person households without children have also become more common. Both groups are disproportionately likely to be at the bottom of the income ladder.
3. Free-wheeling job markets
State ownership of corporations has declined. Price controls have become less common. Minimum wages have fallen relative to average wages. Legal changes have made it easier to fire temporary wokers.
Taken together, these changes have actually improved overall employment levels. (Businesses are more likely to higher hire workers when they can pay lower wages and when it's easier to fire people.)
But despite the gain in employment, the same shifts may also have driven up inequality. In the words of the report, "the high-skilled reaped more benefits from a more dynamic economy."