The product info sheet says that you will get a gloss coat if you put it on thick, something to do with the way it sets or hardens, and the thicker coat takes longer and so appears glossy.
The actual wording on the last page is "Avoid heavy coats as this will result in high gloss levels."
Another possibility is that the tech sheet says to thin and clean up with turps, not thinners.
Maximum thinning is 15% turps.