Following article from the New York Times may help clear this up. "Light" olive oil is evidently a marketing creation and is accomplished by tampering -- processing, mixing with other oils, etc. -- with natural olive oil.
This article per se is a bit vague on the "how's" of light oil, but a quick Google search will bring up a couple more articles that make it clear. Either way, reading the fine print on the label sounds like a good idea if you want to ensure you're getting 100% olive oil!