@User @User The reason for asymmetric rules for metal-via overlap is because of corner rounding, line-end shortening and overlay errors. Lithography is in first order a low-pass spatial filter of your design, thus square corners in your design will be rounded. The spatial filtering may even lead to line-end shortening that can't be fully corrected with so called optical proximity correction (OPC). Additionally when you have two layers on top of each other the alignment will not be ideal so you get so called overlay errors. Asymmetric metal-via rules are then meant to have enough overlap between the via and the metal for yield reasons taking into account these production imperfections.