When you press Alt while targeting an ability, the squares that ability will hit will light up. However, some abilities will light squares that they will not actually hit. A very common one is Zuki's first ability splash damage. I've also noticed it on Orion's first ability, and it probably happens with other freelancers too. Adding an image so you can see that some squares light up even though they will not be hit.

Edit: Not sure if image is showing, here is link: https://image.ibb.co/gPyGZ6/artargetbug.jpg