If U "got it", then ignore this.
The object/the actual reticle itself has crosshairs on it that are straight as hell.
What you're seeing at the back end of an optic is something different/a different thing, which is a projected image of the reticle that suffers from "curvature of field" where the image is curved along w/any lines in the image like a set of crosshairs.
If you look at a circular object like a quarter from a particular angle U can see its curvature.
If you stand a quarter on its side, look directly above it, from that angle you don't see any curves, but something that appears "straight" across its width.
Both images are showing the same circular coin, only one is showing its curvature.
Think of the outer edge of the quarter as a crosshair, like the crosshairs of a reticle and the same thing/same principle (that looking at something from a different angle can make it appear straight
or curved) applies to eye placement and/or a cellphone you've placed behind the scope re what U see thru the scope.
The irony is that even though you're seeing a projected image at the back of the scope that has some degree of curvature of field, the crosshairs you see will appear "straight" if your eye is centered and it's optical axis has merged with the centerline optical axis of the scope.
Said another way, your eye isn't at angle where you can see the curvature of the crosshairs, and believe me, the crosshairs are
still curved, you're just not seeing it, so it appears straight like the crosshairs in the actual reticle.
The image by the OP at the top of this discussion shows the curve of the crosshairs when either your eye or a cellphone is off axis and not centered thus exposing what curvature of field does to the crosshairs in the projected image.