Tilted upper cabinets due to hanging rail?

The assembly instructions for the wall cabinets show both mounting options – with or without rail. Without the rail, plastic parts are inserted that create the distance to the wall. Then, together with the lower spacers, a distance to the wall is created so that the cabinet is ventilated from behind.

I find the assembly with rails much better because you can compensate for "waves" in the wall by shimming the rail, which makes the wall cabinets align at the top from the start. I mounted our kitchen with rails because the walls were not exactly straight; you just have to adjust the lower spacers a bit afterward, i.e., partly omit them and/or install a suitable small board or similar at the bottom, otherwise the lower alignment of the cabinets is not correct.

Dear knowledgeable ones! I would like to revisit this discussion. I have to install an Ikea corner cabinet METOD. On one side it hangs on the Ikea rail, on the other side - due to the nature of the wall - on a screw. No matter how I measure: the holes for the rail and the screw are in the right place, but the cabinet is crooked towards the corner at the back. The first idea was to "rebuild" the missing rail on one side with a piece of wood to act as a spacer. Unfortunately, that does not solve the problem. I can't figure it out: even on the side with the wall without a rail, I attached a spacer at the bottom - one was missing. Maybe that needs to be removed? It would be nice if someone understands this for me and explains it to me. Best regards from Hesse, Ninine