path doesn't go through a point...why?

I drew this with TikZ and I don't understand why the path I drew doesn't go through O:

When I compile, here's what I get:



and when I zoom in on O:



As you can see the path doesn't go through O and I don't know why... Is it my code?

The issue can be solved by replacing \filldraw by \fill. This is illustrated below. The command \filldraw will also draw with a certain line width and this extra width causes this effect.