The lower swale underneath the same tree, collected water too. From both images I can tell it is definitely lower on one side, so the contour is not level. It is what I suspected as I was digging, but it was good to have it verified.
I would have dug deeper on the far side, except I had a few tree trunks that we chopped down getting in the way. I would have to chop away at them with an axe, which is not something I wanted to do. But I think these swales hold enough water, worth the exercise of digging them.
All that is left to do now is to mulch over the top and wait for the settling period. It will eventually even out on contour, when silt and debris fall into the lower spots.
There are more swales to be dug, but I'm happy with what I've done for now.