There isn't a correction for the pattern and it has been tested several times. I have also taught the class quite a bit and have noticed that there are a lot of size variations that come out of this pattern, just based on all the different styles of workmanship, the bias edges, the funky angles, etc. In addition, I have gotten several emails with the same comments - mainly that the trunk sections are turning out longer than the tree sections. There are a few different things that could be causing the trouble:
#1 - There are 16 seams in the section with the trees and only 8 in the section with the trunks, so if you are off even a little bit with the seam allowance, that can cause the tree section to be smaller than the trunk section. It sounds like that probably isn't the case with you, but I thought I would mention it.
#2 - The tree section has lots of bias edges and I have found that there is more shrinkage with the bias edges if you use lots of steam (which I do). I found that if I gave the fabric a good steaming before I cut out all the angles, I had less trouble with shrinkage. The trunk section has no bias edges so there is a lot less shrinkage.
#3 - Aligning the angles - If you don't have the angles lined up just perfectly, this can cause your piece to come out a different measurement. You are aligning them right if you have a perfect 1/4" seam allowance above the tips of the trees. If this is off, the section can be larger or smaller.
As you can see, there are quite a few ways to get off! (and this is just to name a few.) If you have already sewn the sections of the runner and have found the tree section to be too larger, it is probably easiest just to take a little deeper seam on the trunk sections so that is turns out a little smaller. That is much easier than adjusting the section with the trees.
If you haven't started the runner, you can always sew the tree section first and measure it before starting the trunk section. The tree section should measure 34 3/4" unfinished. If it is less than that, just cut the tree trunks a little skinner to accommodate that (for example, cut the width a scant 2 1/8" or 2").