If you place the square in the center, you sew two seams in each direction. The two seam allowances take up 1" of fabric, so the block reduces to 9" (8 1/2 finished). If you place the square in a corner, you only do one seam each direction, so you end at 9 1/2" (8" finished). You're not doing anything wrong, that's just how it works. Another vote here from trimming them all to 9" before sashing.
Last edited by Krisb; 01-03-2015 at 11:11 AM.