This is perspective distortion, and it is an inevitable outcome of projecting a three-d world onto a flat surface. See What is the difference between perspective distortion and barrel or pincushion distortion? for some details on different kinds of distortion, and How to correct perspective and geometric distortion? for what you can do about it.

Many people think that this is a property of wide-angle lenses, but that's not actually the case. It's just more apparent because with a wide angle lens you're putting closer objects from, well, a wider angle into the frame. If you change where you stand and use zoom or a different lens to get a similar framing, the perspective distortion will be different. On the other hand, no matter what you do with lenses, there's no way to change the perspective without literally changing your perspective by moving.

Try this: put your head where your camera is and look straight down. If there's an object right below you, you'll see just the top and none of its sides. But if you look over to the left or right, you'll see a bit of the sides of objects a foot or so away from the center — and quite a lot of the sides of objects further than that.

To avoid the problem, you'll need to either:

• move the camera much further away — in your case, much higher — so everything is mostly top-down, or
• move the camera back and forth, getting the top from multiple locations, and then stitching together, or
• use software to stretch things to look like you want them. (Depending on the scene, this may work well enough.)