I used to have the same problem. I did two things to solve it:
I use a 50 weight thread, and if the quilt is a wallhanging, I use 60 weight. The little extra bulk from the thread adds up enough to make a difference.
MAke sure your seams are very flat. Sometimes you may think you pressed well, but when you look at the front of the block, you may notice that the seams and a little folded, but not totally flat. The measurements are given so exact that your pressing has to be perfect in order to achieve the right size.
In the long run, If all theblocks you are making are in the same quilt, it doesn't really matter what size they are, as they will match when you put them together.
Maria