I don't know about garmin but on my lowrance It matters if the the target is moving or not, a moving fish will appear as a line relative to it's size, or rather density. I have caught these deep walleye before, I would guess they are going after bait that didn't make it through the cold, or sometimes it seems like the work under other feeding fish, picking up the stunned and wounded.
I can always tell a drum by the way the vector toward the lure, those suckers can flat out feel the vibration of a falling spoon !