I would say that if your squares are 6 1/4 x 6 1/2 that your seams in your rail blocks are slightly larger than 1/4"
One way to fix would be to unsew and then resew all your rails with a scant 1/4" seam. Use a ruler to check your seam allowances.
Another way to fix, if they really all measure the same is trim 1/4" off the larger side and make them evenly sized. In other words, square them up so that they are a perfect square.
But whatever you do, for the future check the actual seam allowance and move your needle or adjust your fabric to get the 1/4" or scant (a thread short of 1/4")