First, what do we mean by "global inequality"? Well, let's break that down. We're talking about a metric measuring income dispersion, which is, yes, a measure of the gap between rich and poor. A common metric is the Gini Coefficient[1]. And instead of looking at the coefficient of a single country, if we look at the entire population, we get a metric of global inequality. Not hard, right?
Second, what have metrics of global inequality been doing since 1980? Why, they've been falling[2]!
So, yes, the "gap between the rich and poor [has] lessened". I have no idea why you or anyone else might think that it seems otherwise. Find an op-ed or column about the global economy from anytime in the past decade, and you've got a good chance of it either talking about how real incomes in the West (ie, the global 1%) are stagnating, or how real wages in China (ie, the global 99%) are booming. There's really no way this could happen and not result in a significant reduction in the gap between the rich and poor. And indeed, that's exactly what's resulted. (And to tie it back to a perennial HN favourite, the mechanism by which this has happened - an unprecedented reduction in global inequality and a massive reduction in absolute poverty - is exemplified by Apple and Foxconn.)
(You're also right that we could have a reduction in absolute poverty even as global inequality increased. But that's now what is happening.)
[1]: http://en.wikipedia.org/wiki/Gini_coefficient
[2]: I'm resisting supplying citations because a quick Google search will turn up, literally, pages of results. Still, if you want one image, this one[3] isn't bad.
[3]: http://media.economist.com/sites/default/files/cf_images/200...