I normally have the same problem with blocks being slightly different. I even have a 1/4" pressure foot, so you'd think they'd all be the same.
For this mystery quilt (since Bonnie said measurements needed to be exact), I'm making all the blocks a teensy bit larger then trimming down so they all end up the exact same size. I don't have the tri-rec ruler, but my V-block ruler does make the blocks slightly larger for trimming down.