This question comes from Jacob Brunny:
Alot of times I create guides and will drag over boxes or whatever to fit to them and it looks good from like 100% or something like that, then when I zoom in closer I see that they are actually not right up to the guide but off by a small amount, it seems Illustrator never gets them perfect except if you are zoomed in alot?
SIGH. In reality, this has been a personal frustration of mine for many many many years. I touched on this issue here but the upshot is that Illustrator doesn't snap object geometry. Rather, Illustrator snaps cursor position.
Go ahead and try it. Draw a guide. Then draw a rectangle and fill it with a color. The click anywhere in the fill area of the rectangle (not on the path), and drag it towards the guide. The edge of the rectangle will not snap to the guide. Keep dragging until your cursor touches the guide -- and you'll see that the cursor itself does snap to the guide.
So where you click on an object makes a difference on how that object snaps. If you click on an anchor, it appears that anchor snaps to another anchor when you drag it, but remember that it's the cursor position, not the anchor. Since when you zoom in really close, it's easier to mouse down on the anchor or the area you're looking to snap, it appears as though Illustrator is more accurate when you're zoomed in that close.
In InDesign, object boundaries DO snap to guides. You can grab a picture frame anywhere and drag it -- and you'll see the boundary of the frame snap to guides.
Because a typical Illustrator document contains many more objects than an InDesign one does, one might think that having objects snap in Illustrator might not be wanted. But at the same time, I'm not happy with the current implementation either. So instead, we struggle with this every day.