Several years ago I had bought some graphed template plastic. I was happily making templates and my square blocks weren't fitting together as nicely as I expected them to.
When I checked out my template, it wasn't square - even though I had very nicely cut along the "proper" lines.
Much to my consternation, it wasn't printed evenly.
When I measured the grids in one direction, I got one number. When I measured the grid in another direction, I got another number.
I'd say over the width of the 8.5 x 11 inch sheet, there was an 1/8 inch variation.
Point of this note - measure the graphs/grids before using just to make sure that the measurements are true.
It sounds trivial, but 1/64 inch variation (especially if there are many seams) multiplied by pattern inaccuracies, cutting/measuring variations, and seam width variation can add up to a noticeably different finished size from what the pattern said.