I don't know anything about tank guns. However, just from how badly the 96B failed in the shooting contest, it doesn't sound like something related to the design or fundamental quality of the gun.
If the 96B consistently shoots about 10-20, even 30% worse than other types of tanks, then it is possible that the 96B's main gun has an inferior design and quality. PLA might have, for some reason, allowed their main gun to underperform to some extent.
Yet, the particular 96B in question missed the targets so badly. I don't see how the PLA would allow something so inferior to be installed onto their backbone tanks. And I also don't see how the PLA would send tanks with such inferior design abroad to a major tank competition, where everyone is watching. Such face-losing would be unthinkable...
In other words, the main gun of 96B should have performed to the PLA's satisfaction in all their tests. And it would be impossible that PLA's goal for their backbone tank's main gun is "to only miss the target by X meters"...
So what happened at the competition? No clue. The most likely explanation to me would be an unexpected equipment failure / malfunction...