OK. We are not building rockets here. The "smidge" of difference between the ruler and mat are so small I doubt if it will ruin a quilt. The difference between the ruler and mat cannot be that bad or you would have noticed it before you cut. If you cut the whole quilt the same way, they will all be the same "variance" so won't matter anyway.
Sometimes we seem to be making MOUNTAINS out of little bitty molehills. If you get this hung up on a quilt, you need to relax a bit. Really sloppy cutting is the stuff that makes the pattern pieces not fit right.
Same goes for the 1/4" seam rule. I used to do all my quilt seams with 3/8" seams, because that was the width of my pressure foot on my very old Singer. Nobody noticed. Since all the seams were the same it still fit together, just a tiny bit smaller when finished. My daughter is still using that 35 yr old quilt!