You know how you can make a half-circle/half-oval by setting it to 180 degrees? For some strange reason, Hammer's behavior when creating incomplete arches is erratic and dependent on that checkbox.
Figure 1 was created by drawing a square representing the imaginary full circle, with that setting unchecked.
Figure 2 was created by drawing the same square next to it, with the setting checked.
Figure 3 was created by drawing a half-square with the setting checked.
You would think figure 3 would be correct and figure 2 would at least have an even thickness. I personally can't imagine a scenario where you would desperately need them not to. So my advice is to leave the box unchecked.