So viewing from 40cm away is about the point where 220PPI crosses the point where you can tell any difference? That makes me think 220 is quite a well picked number, as that's about how far away someone might typically be viewing.
There is a difference between your ability to resolve, and whether you can tell _any_ difference. The usual measure of sight is visual acuity, which is your ability to discern details of a certain size at a certain distance. Specifically, in most exams, it's the ability to tell strokes from blobs.
However, even when the pixel density is beyond your ability to discern details, they can still affect your perception in other ways. For example, misalignment of borders can be detected far beyond what your visual acuity is.
On computer screens, particularly, pixel density also affects how things are rendered, think diagonal lines, curves, hollow spaces, etc. Take an extreme example, a box rendered on a 4x4 screen is going to look better than on a 2x2 screen even if the former is twice the distance away.