You first draw two circles when different radii.
When you select two point over the circumference, and you connect a line in between these points and the center of the circle, you will always obtain two sides with the same length. It is because the length of these sides coincides witht the ratio of the circle.
Then, when you connect the points over the circumference between them, you have a side that can have a different length compared with the lengths of the lines connected to the center. Thus, you obtain an isosceles triangle; you have two sides with the same length (remember, it's the same as the radius) and one side with another length.