Why is there an difference in measurements between rotation in same spot?

The rod bubbles have difference tolerances. Depending on the height above the point; the measurement can be different due to the level bubble error.

Carbon fiber rods can see up to a 40 minute error.

Inverted rod can see up to 20 minute error.

These errors fluctuate based on their height above the point, and can have the calculated measurements error in the table attached.