No the middle should not be at Y = 50.
It's only an approxiomation, but the center which you almost always want in a 3D object is the center of mass, and the locations of the vertices gives you an approximation of that.
The reason you want the center of mass is because that is the point an object in the real world is most likely to rotate around.
In your triangle example, the two vertices at 100 Y means that that is where the wide end of the triangle is, and thus the most mass, so the center of mass is closer to them than the other point.
It's only an approxiomation, but the center which you almost always want in a 3D object is the center of mass, and the locations of the vertices gives you an approximation of that.
The reason you want the center of mass is because that is the point an object in the real world is most likely to rotate around.
In your triangle example, the two vertices at 100 Y means that that is where the wide end of the triangle is, and thus the most mass, so the center of mass is closer to them than the other point.