How does the damage of Zarya's weapon work?

Hopping into a quickplay game (with custom settings) showed that on average, when firing at 0 charge, and then not increasing my charge - resulted in an average damage delt per ball of 24 (I got 24 3 times, and 23 once, so I probably goofed up that one shot).

Now firing a ball then taking damage to my barrier (to increase the damage a ball did), resulted in an average of 33 damage delt per ball (with 3 balls delivering 33 damage, and one dealing 32).

So, yes, the charge actually does change as the ball flies.