The center of the Modeling Window is not necessarily the center of the model. You need to draw the circle from the intersection of the grid lines.
You can draw the circle from its center by pressing the Option or Alt key. Pressing the Shift key will constrain the oval to a circle.
Turn on the Snap to Grid option in the Edit Menu, and then place the pointer at the center point of the model, the intersection of the blue and red grid lines. With Shift-Option keys pressed, drag out the circle from its center.