Here is whats going on with the "dead zone".
Because every return within the cone gets drawn on the screen at the same time, things like fish, that fall in between the shallowest and the deepest parts of the bottom, are hidden by the bottom return when they get drawn on the screen.
In this example, imagine you are trolling along parallel to a drop off and there are a total of 6 fish under the boat at various places. (Im just making up these numbers to make it easy to explain)
Lets look at the 77 deg side first.
The wider 77 deg cone hits the bottom at 20 ft on the shallow side and 40 ft on the deep side. Notice that bottom line is drawn starting at the 20 ft depth extending down to the 40 ft mark. That difference in depth between the hi and low points within the cone creates a 20 ft wide "dead zone" where fish wont show up. The fish returns are there, but they are hidden within the bottom return because all the returns are drawn at the same time on the same vertical line at same time on the right side of the screen.
There are 6 fish under the boat within the 77 cone, but only the green one shows on the screen. It happens to be above the shallowest part of the bottom thats within the cone, so its arch can be seen on the screen. The other 5 fish fall within the dead zone and cant be seen on the screen.
You can see on the left side, that the green fish is actually about 7 ft vertically above the bottom, yet it gets drawn on the screen so that it looks like its almost sitting on the bottom.
Also, the digital depth number in the top left is 27 ft in my made up example, but the bottom line is being drawn on the screen at 20 ft. Thats because the 20 ft mark is the shallowest point within the cone. The digital depth number comes from some sort of averaging/calculation that the MFD does when it decides what the "depth" is based on all those different returns. Unless the bottom is dead flat those numbers will almost never match.
Now the 200 side.
The narrower cone on the 200 side hits the bottom at 25 ft on the shallow side and 30 ft on the deep side making for a much narrower dead zone. There are two fish within the 200 cone, but once again, only the red one shows on the screen. Because of the smaller dead zone, the 200's narrower cone is able to "see" fish that are closer to the bottom than the wider cone on the 77 side. It also comes closer to showing how far up from the bottom the fish really is. That narrower cone also draws the bottom depth closer to what is really is under the boat and will also give a better representation of the shape of the bottom contour as you drive over it.
The main downside to the narrower cone is you miss more fish that are off to the side of the boat. You pretty much have to drive right over the top of them to see them where as the wide cone is more like a shot gun blast and hits anything even if its off to the side.