Some rulers have fat lines to indicate the measurement markings and you need to be aware of how YOUR ruler is marked. On mine, for example, I make sure that the edge of the fabric is smack-dab in the very center of the fat marking line. What they are referring to is that their measurement apparently includes the marking line for precise cutting.
If you are unsure about your ruler, cut 2 or 3 scrap pieces by aligning the marking line on the edge of the fabric differently. One on the inner edge, one on the outer and one in the middle. Then measure your fabric strips to see which one comes closest to your ACTUAL size.
For me, it makes no never-mind because I don't plan my quilts to be a certain size. So for me, consistency across the blocks is more important. However, just think how a tiny difference can become a huge difference when it is repeated over all the blocks.