Audi evidently didn’t want to give anything to the Mercedes E-class in the shutline and craftsmanship battle.
The W-126 had a visible weld crease under its rear lamp. So Audi spent a bit extra to avoid it. 20 years later the C4 Audi A6 is still an object lesson in the pursuit of orderly detailing. The only line visible is one related to the boot aperture.
[Editor’s note: Text altered to correct the A6’s model designation – see comments below]