I can think of two reasons. One, that specific point is worth enough that a half point move would be too much. These often occur with the football 3 and 7, but it could happen with basketball points too. Oly was dealing MIA +6 -105 and DET -6 -115 for a while tonight, which isn't quite as drastic as moving from -6 to -6.5. Two, the book doesn't want to get middled, so they move the juice instead of the spread or total.
