1. if a itemframe with a compass inside is on top of a block, the compass is pointing 90° away from spawn. By turning the compass 6 times it gets to point at spawn.
2. if a itemframe with a compass inside is placed on the bottom of a block, the Compass can point in any angle from spawn depeding on position of the block.

The 1. could be fixed by turning thedefault Rotation of the itemframe but this wouldnt help the 2.