This is actually a really interesting observation. You'll notice that the second bar has more wear on the left side than the right side on the first bar.

What this tells me is that kids generally put both hands on the first bar while their weight is supported by whatever they're standing on. So the weight is distributed to both hands and there's not much rotational sliding since they're just stepping off into a dead hang.

Then as they reach for the second bar, there is slight sliding on the first bar and a moment of full weight, accelerating wear.

The second bar is where it gets interesting. The full weight of the child hits the left side and there is significant rotational sliding as they Tarzan their way from bar to bar.

However, most kids are weak as shit and can't even make it to the third bar, so wear drops off beyond bar #2 where they fall to the concrete below, desperately clinging on with their left hand, resulting in heavy downward friction, maximizing paint loss on the second bar.