Your aperture is too wide for the shooting distance, focal length, and the magnification you are using to view your image. As a result only the center parts of the cake appear sharp and the extremes of the cake, both near and far, are slightly out of focus.
There's only one distance that will be in sharpest focus. Everything closer or further away from the camera will be blurry to one degree or another. The further away from the point of focus, the blurrier things will be. What we call depth of field is the distance within the area where the blur is not noticeable to our eyes. There's no magical line at which things on one side are equally sharp and on the other side are equally blurry.¹ It is a gradual transition from sharp to blurry. There are several variables that affect just how far from the point of focus things start becoming noticeably blurry to our eyes.
The blur becomes noticeable at smaller distances from the point of focus if we:
- Use a longer focal length/narrower angle of view
- Use a shorter subject distance
- Use a wider aperture
- Use a larger display size
- View the displayed image from a closer distance
- Have better vision
The blur increases more gradually at larger distances from the point of focus if we:
- Use a shorter focal length/wider angle of view
- Use a longer subject distance
- Use a narrower aperture
- Use a smaller display size
- View the displayed image from a larger distance
- Have weaker vision
To properly calculate depth of field all of these factors must be taken into account. Many DoF calculators make (often unspoken) assumptions about some of them. Most DoF calculators, such as DOF Master, assume an 8x10 display size viewed from a distance of 10 inches by a person with 20/20 vision. Suppose one properly calculates the DoF from such a chart and then displays the image at both an 8x10 and a 16x20 size viewed side-by-side. The 8x10 print should demonstrate the calculated DoF, but the 16x20 print, viewed from the same distance, will have half the DoF of the 8x10 print!
How can this be?
In a way, depth-of-field is an illusion. There is only one plane of focus. Everything in front of or behind the point of focus is out of focus to one degree or another. What we call DoF is the area where things look, to our eyes, like they are in focus. This is based on the ability of the human eye to resolve certain minute differences at a particular distance. If the slightly out-of-focus blur is smaller than our eye's capability to resolve the detail then it appears to be in focus. When you magnify a portion of an image by making it larger or moving closer to it you allow your eye to see details that before were too close together to be seen by your eyes as separate pieces of the image.
Since things are gradually blurrier the further they are from the point of focus, as you gradually magnify the image the perceived depth of field gets narrower as the near and far points where your eyes can resolve fine details moves closer to the focal plane.
Keep in mind that when you are "pixel peeping" on your computer at 100% display, where one pixel in the image is displayed using one pixel on your monitor, you are viewing at the same magnification as if you are displaying the full image at a very large size. A 24MP image viewed at 100% on a 23" HD (1920x1080) monitor is the equivalent of a piece of a 60x40" print!
Here's an online depth of field calculator that gives the user the option of indicating display size and selecting desired visual acuity as well as entering focal length, aperture, and focus distance. Just click on the show advanced link to access all of the features.
¹Technically, if the blur is smaller than the resolution limit of the image sensor, then things will be equally sharp/blurry. But the pixel pitches of the vast majority of current digital sensors in cameras intended for creative photography are about 1/3 to 1/7 the width of the minimum blur we can see viewing an image created with those sensors at 8x10 display size from 10 inches away.