Because you discharged it for 9hrs.
I couldn't find that looking on a tiny screen.
310Ah x 51.2V = 15,872Wh
So your discharge to 13% but recovered to 40% on its own, lets go with 60% discharge and run some numbers.
15,872Wh x .6 = 9523Wh discharged.
31Ah x 51.2V x 9h = 14,284Wh (using...