Why does this happen? When I defocus a star with a 25mm eyepiece, the donut doesn't look circular. It looks oval.

But when I'm doing a star test, as everyone recommends a high power eyepiece, I used high power. It looks circular this way.

So, what's wrong? My Explore scientific 8 inch Dobsonian came with the secondary slightly offset in a way that when I look through the collimation cap, the secondary and the focuser tube are not concentric. But the secondary looks circular.

If I try to center the secondary, it starts to look oval instead of circular from the hole of collimation cap.

Do you people with Dobs get the circular donut with all eyepieces or not? Help me.