I have a shape.
Some points of it have handles, some are not.
For example, two points adjacent to the the bottom-most one: the right one has no handles, the left one has one.
This, for some user-unfriendly reasons, matters and dictates Ai to draw an extension or not to.
If I grab bottom point and try to move it along the right segment’s virtual extension, AI draws it for me and I can snap the point to it — 'because the adjacent point has no handles'.
If I move this same point along the basically same straight left segment, Ai shows me nothing — 'because the next point to the left has a handle'!
This is a difference that is purely made up and has no valid reason to exist.

I try the right-center point then. As you can guess, Ai draws a guide if you move the point along right segment’s extension, and ignores the left one — 'because it has a handle'.

All of these segments are straight! There should be no difference between them, but it exists now.