The question is: did you count rows? Or were there instructions that you followed that made the size come out?
If you did count the rows, then just make sure that in sewing the pieces together you attach one row to one row. That way things are even in the end.
If it is immensely different, I would worry. If not: oh, well.
I do go by the measure when matching the parts, as well. You just have to make sure that you're not stretching the piece while measuring it.