Sonar arches.
Simply put, sonar measures distance. Hold that thought for later.
Assuming you're stting at a chair in your home reading this, look up and pick out the spot on the nearest wall directly in front of you. Measure or estimate the distance (from your nose). Let's say it's 5 feet. Now, facing that spot directly, raise your arms toward the wall and spread them to approximate a 20 degree angle. Mark a spot that either arm points to and measure that. Let's say it's 5 1/2 feet, a difference of 6 inches.
Now turn and do the same for the farthest wall, and note that the difference in distance between straight on and 10 degrees off is greater than the near wall. Imagine the scenario out to greater distances of 20 and 40 feet, and the difference grows even more.
Now translate that into a sonar beam on the lake. As your boat approaches a fish at the edge of the cone the sonar marks a distance on the display. As you move directly over the fish it continues to update that distance, which is not only some measure closer, but reflects a stronger signal as well. Then, as you move past the fish, the signal gets further away and weaker at the trailing edge of the cone.
The distance difference for shallow fish is small, rendering little or no discernable arch. The deeper the fish, the greater the distance difference, and the greater the arch. You can manipulate it a bit with your zoom. Say your screen is set to display 40 feet. A distance difference of 6 inches doesn't make much of a showing there. Zoom in to a 5 foot window, and 6 inches is pretty obvious.
Now factor in all the variables where a fish might not be sitting still, or directly under the xdcr. It's still all a matter of distance at a frozen moment in time.
Added
It's important to note that if your xdcr isn't pointing down exactly at 90 degrees from the surface this all goes out the window pretty fast. If you're marking fish that all appear as angled straight lines, you need to re-aim your xdcr. If the marks angle upward, move the xdcr back and vice versa.