You may think it is laterally balanced but the model is giving one indication that it is not. Imagine the right tip is heavy, so there is a tiny amount of left aileron trim to compensate. The two forces cancel each other out. When inverted the left aileron trim is still left aileron trim. But the heavy tip is now on the left side so the two forces add to one another instead of cancelling out, and the model rolls left.

Weight is not an intrinsic property of matter, mass is. Weight is mass acted on by an acceleration, so the greater the acceleration, the greater the difference in weight between the two tips. You can cause more acceleration, and thus greater weight difference by pulling up into a loop. Since the aileron trim will be constant assuming the speed remains constant, if there is a weight difference between the tips the model will roll off to the heavier side. If it pulls off to the right during a pull up, and goes off to the leftt if you push up from inverted flight, it is a lateral imbalance with the right side being the heavier side.

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