When you sew a quarter inch seam allowance you are using up 1/4 of an inch from the two fabrics you are sewing together, so you lose a whole 1/2 inch. So if I sew two 3" strips together, I will lose a 1/4 inch off of each (you can see this when you sew, to the right of your sew line you have 1/4 inch of the two fabrics) thus ending up with a 5.5 inch strip. So when I sew three strips together I lose a 1/2 inch from the first two sewn together, then another 1/2 inch when I add in the third piece. Then when I eventually sew that to other blocks I will lose a 1/4 inch on each side, thus another 1/2 inch total.
So I sew a 2 to a 3, I end up with 4.5, sew the 4.5 to a 2, end up with 6, sew that to other 6s I get 5.5 finished blocks.
Rachel