Basically the UV coordinates start from edge to edge, that's why first pixel center has a value of 0.5/dimensions and pixel coordinates start from the center of first pixel.
Yep! I get ya. I haven't gotten to it just yet due to some other priorities right now for this time of the year but I'll get to it! In this case, After Effect's built in sampler function considers the origin of the pixel at the top-left of the Texel. And it just needs a (0.5, 0.5) texel-offset (0.5/width, 0.5/height).
https://ae-plugins.docsforadobe.dev/effect-details/graphics-utility-suites.html#...