I'm going to guess that someone tilled the soil prior to seeding or sodding. If so, it will be forever lumpy if left alone. Tilling is commonplace due to the fact that just about every source for lawn preparation tells you to do it. The problem with tilling is getting the density of the soil the same everywhere afterwards. If the tiller goes a little deeper in one spot, then the soil will be fluffier at a deeper level right there. When the soil settles, it will settle lower and give you a "hole." If you really have to level the soil in preparation, the only satisfactory tool is a landscaper's box blade pulled behind a real tractor (not a bobcat or a skidsteer). The problem with the bobcat and skidsteer is they short wheelbase and high center of gravity amplify the rocking motion of the machine when it hits a high or low spot. A tractor will roll over and actually smooth out the little hills and valleys. But that's all water under the bridge for you.

One relatively easy approach to leveling is to run a plug aerator over the lawn and sweep the plugs from the high spots to the law spots to melt. Run it much more heavily over the high areas and don't run it at all over the low spots. You can do this three times a year to level your lawn without really harming anything.