it's not possible to have either subpixel vertex precision or perspective correct mapping with the PS1 GPU, as it only takes 2D whole-pixel coordinates for triangle vertices. (contrary to popular belief, N64 also uses exclusively fixed point for graphics btw, it just has subpixel units.) better tessellation can mitigate the perspective issues by a lot, but the vertex snapping is unsolvable, and it is indeed present here. look closer and you might see it.

Interesting!

I guess you could pretend to have sub-pixel precision on the PS1, if you did it manually? Eg change the colours around 'between pixels' or something like that?

But that would probably get very expensive very soon.