Sometimes there is a tradeoff between what looks well, and what works well. On the slide shown, the icons with borders have a more regular outline and will therefore look better, especially if they are displayed in a row on the display.

However our goal is icons that are immediately recognizable, and one of the ways to make them recognizable is by having a unique outline. Without the borders each of these icons has a unique outline and will be more quickly picked out of a list.