This is a drawing I created indicating a front sight post, with a target center above it (as if you were shooting a 6 o'clock hold, where the top of the front sight post just touches the bottom of the black circle on the target).

I did not move either the front post, or the target.  I just blurred the image more and more to simulate your eye's loss of focus, which makes the target and the front sight look fatter than they actually are.  As you can see, the target on the left side of the page is above the front sight post, but the target on the right is almost touching.  In reality, there is no difference in the position of the target left versus right, it's just that your eyes fool you when they lose focus.  Your ability to detect that thin white sliver between the target and the front sight is lost if either:

1.  The light dims and your eyes dilate, reducing their ability to focus clearly.

2.  Your eye gets fatigued, and has trouble focussing consistently.

If you are presbiopic (old eyes), so your lens is straining to focus on the front sight, and cannot focus repeatably (especially at the end of the day, at 600), or if brightness is changing, the apparent size of your front sight and the target will be inconsistent, resulting in elevation changes.  You don't typically have windage problems, since both edges of the front sight get equally fat, so the center of the sight remains unchanged.