Three basic things can make your block come out the wrong size.
1) make sure you are cutting accurately. Use the same ruler (or at least the same brand ruler) when cutting the whole quilt
2) You said your seams are the correct width, it's always good to double check. Here's a good tutorial:
http://www.quiltingboard.com/tutoria...ce-t89997.html
if you use a different thread or different weight fabric, you may have to adjust your seam.
3) finally, you need to press your patches carefully. Sew the seam, press the seam to set it, then carefully press to one side from the front. Make sure you don't press a tiny crease in your seam. I press each seam as I go.
The only other thing I can think of is an error in the pattern. I would second the suggestion to PM the other member who made the quilt to see if there was a typo in the pattern.
Good luck!!