human brains appear to be quite good at picking up on subtle things like this, especially in graphics, since we're visually oriented creatures.
And with practice, you get even better at it.
I think its also important to remember that we who stare at pixels all day get pretty darn good at it, a lot more so then the average person.
Also, it bugs us a lot more, because we know how smooth it _can_ look
Same thing with color depths, that whole "True color, the human eye can not see more then 24 bit color" is just marketing, the RGB space cover only a part of our color vision, and someone trained in looking at computer graphics and finding faults easily notice all the banding and dithering going on to try to compensate.
Not saying you shouldn't try to make it as smooth as possible, of course that is a priority (and usually your job as a low level graphics guy)
Really interesting thread!
Getting your delta time right is more work then one would think