Are you measuring the actual seam allowance, or are you measuring the block itself after you sew it, to see if you are getting the correct block size? There is a difference, and the more accurate way to measure your seam allowance is to sew 2 blocks together and measure the finished size, which should equal the sum of the two blocks minus half an inch.
In other words, if you sew together two 2" blocks, your finished patch, measured across the right side of the fabric, should measure 3.5". 2" + 2" = 4", minus half an inch, is 3.5".
Also the thickness of your thread will make a difference, especially if you're "using up" some 40 wt.
I did have an Olfa ruler that was 1/16th inch too wide. Fortunately I had been using it to draw and spotted the flaw before I used it for quilting.