I think its +3 balls for %50 or more team damage and follows like 30-20 or something.
If your team didnt do %50 of the total u wont get +3
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I believe the team contribution bar is either broken or drawn using a different metric than the total damage. Either way I have found it doesn't scale to the actual damage done.
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^
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The bars are not proportional to the amount of damage that was dealt, because Niantic.