Are those all the same petunia, sown and planted at the same time? If so it seems odd that the right half is so much healthier. I would have to guess there is some sort of environmental difference in the sun or soil?

My experience with tidal waves is the more you pinch them back the bushier the get. I pinch off every runner the petunia first sends out (usually four) and then often pinch off the next ones too, so that I have 10-12 runners off of each plant. Then I just let them go to town, and only cut them back later in the summer when the get straggley. But Tidal wave I have mostly grown in baskets.

I also pinch off the first few buds, and try to let the petunia get a few more leaves or runners out first.

This might be something that the ones on left can benefit from, but the ones on right seem OK for now.

It does seem like the right side has a bit of slope (but it could just be my eyes playing tricks), so it's possible they are getting the bulk of water.