On the fineness of stroke---I think that it should in fact be possible to get it as fine as one pixel. Accuracy, though, is another matter. I think that some of these more sophisticated devices might have different mechanisms and algorithms for determining pen point location. As I recall, at least some of these devices with external pickups use infrared or sonar or somesuch for detecting wher the nib is located. This completely bypasses the touchscreen for that determination. Then if the nib is at the exact center of the activation zone, one could theoretically get one pixel accuracy.
I also think that even using the touch screen as the location sensor there might be differences in mathematical algorithms for determining just where the nib is located.
But all that and around three bucks will get you a latte at Starbucks: seeing (and experiencing, I might add) is believing!