Well, dividing an odd number of pixels by an even number still leaves 0.5 pixels as the remainder, you know. If your anchor point is at such an odd value, you'd still get sub-pixel positioning.... Just quantizing the position is not always enough. Likewise, using scaling or effects can still use sub-sampling. There's just a million reasons whyy this could be happening...
To have edges exactly line up on the pixel grid the dimensions must be known and the placement of the edges must be accurate. If you are using vector art from Illustrator, unless you turn on snap to pixel when the art is created the lines can be anywhere. You also have to be working in square pixel comps with square pixel artwork. Make sure the settings are correct. If your comp size is whole numbers and your artwork is odd numbers then the position must be on the half pixel or the anchor point on the even pixel. The opposite applies. BTW odd numbers in the composition settings mean that any MPEG or h.264 compression will resize the render and foul up all of your fine lines. The moral of the story is to use standard composition sizes and standard frame rates for all of your projects unless your client has a specific requirement for something else. Nested comps (pre-comps) can be trimmed to any comp size but those should always be even pixels.