that must be one of the reasons we are always told to use the same ruler & cutting mat through out a project- although I must say- I have numerous rulers that do 'measure' correctly- lines may not be all the same- one has 1/8" marks, other 1/4" marks, a couple have different angles marked- but inch by inch (& 1/2") do line up- except the one that has been used so much the edges are about worn off & really needs to be replaced. but I've seen many, many tutorials that state to always use the same ruler through out a project- to avoid any discrepancy.