I'm 100% certain that you both have small hairline scratches on the screen.
They are only visible if you tilt it into light after you have polished the screen to remove dirt and dust but they will be there.
And they are the most annoying because you thought you kept it scratch free.
Never used a protector on my 800 and don't have any scratches either. If I do like you say they must be that small that I cannot see them; even after cleaning and tilting the screen. And if I can see them after a year of use; I don't really care.