measuring the seam is not the best way to judge your seam alloowance. you need to measure the units sewn together to see if you are hitting it. I have dozens of 3.5" unfinished 9 patches I've sewen over the years to check seam allowance over the years on different machines. .
I cut 1.5" strips (pressed fabric, unpressed fabric grows when pressed after sewing), sew 3 strips together, slice into 1.5" strips again and make your 9 patch and press. If at that point you have an exact 3.5" square you seam allowance is on the sweet spot.