It's hard to talk about something so based in individual perception: it's true, my eyes could literally be different.

However, my strong suspicion is that the visionOS native apps have a larger font size and bigger, smoother UX elements. In this case, the mind doesn't notice a little blurriness around the edges.

Whereas with a mirrored display, there's tons of tiny thin lines where it's easier to notice that visual acuity is not 100%.

Some people immediately notice 90/120Hz screns vs 60. Some will see pixels on a 3K 14" screen while it will "good enough" for most.

Aside from sheer eye sight, I think there's also attention and sensibility to these specific things. The same way web designers recognize fonts from 4 letters when generic users barely pay attention to whether it's serif.

People’s eyes vary quite a bit even with corrective lenses.

Some people have much worse vision? Makes sense that people would perceive visual clarity differently