As anyone who has tried to bend a structural angle has experienced, it is difficult to roll the member perfectly square. Angle bending causes the metal section to twist because the easiest way for it to bend (the weakest section modulus and moment of inertia) is either apex-out or apex-in and not leg-out or leg-in.
Consider what is happening when we try to bend an angle leg-out. The vertical leg of the angle is being rolled the “easy way;” the horizontal leg the “hard way.” But because the two legs are connected, the center of mass / neutral-axis is somewhere between the two legs. Any material inside the neutral-axis undergoes compression. Any material outside the neutral-axis sees tension or stretching. The stretching is limited to about 25% elongation or strain. The compression limit is trickier to define because it will buckle / ripple long before the ultimate compressive strain of the material.
Leg-in angles are the hardest to roll, partly because the thin innermost edge is under the most compression (because it is farthest away from the neutral axis). Often we see an offset in the vertical leg.
With the proper machinery, tooling and operator, however, we can avoid all this bad behavior and end up with perfectly square rings.
Rolled angle rings are often used as flanges to connect cylinders or pipe and can be supplied with or without bolt holes in either or both legs.