Synchronous Generator Open Circuit Test
The open circuit voltage is a result of the Magnetic Flux Density (B) and the Magnetic Field Strength (H) results from the applied current.

Increasing the current leads to a higher density as can be seen in your picture (The blue line) and in the segment g-b of the BH curve. Decreasing the current leads to a lower density as can be seen in your picture (The orange line) and in the segment b-d of the BH curve. 

The lines in your curve are more or less linear due to the fact that the exitation does not reach the non linear sections of the BH curve.

Therefore your assumption that the difference in the lines are a result of the machine hysteresis is correct.