That's a common misconception. That acuity calculation is based on knowing nothing about the image; imagine applying it to arbitrary noise. When you have things like lines and edges your eyes can pick out differences an order of magnitude finer.
It's always been a weird topic - science (appears to, due to the aforementioned misconception) say one thing, and yet I have eyes and see a difference that the science says I shouldn't see.
https://en.wikipedia.org/wiki/Hyperacuity