User blog comment:RandomGuy96/So how do planetary defense cannons work?/@comment-5379655-20130128042915/@comment-4721065-20130128100546

Maybe the ships simply cannot detect the projectile at the early stages due to the planet's magnetosphere, gravity well and everything else masking the slug and when they notice there's a chance that they won't have time to move out? That, and maybe the AA guns' fire in specific patterns to maximise the chance of hitting the ships: i.e. you have 7 guns, one of them fires directly at ship's position and the others' fire to the left, right, up and down of the ship, as well as ahead and behind it (assuming it's possible for ME ships to reverse that fast). The AA guns have their own VIs, and so if they take the ship's average speed and everything else into account they might well be an effective (if not efficient) counter to supplement defensive fleets.

Regarding the frigates & smaller ships from the other discussion, maybe their projectiles simply aren't large enough to survive entering the atmosphere and so they will always burn up before they get the chance to hurt the AA guns?