Trouble with crochet gauge?

I've only crocheted afghans, scarves and hats so far. I would now like to try crocheting tops and cardigans. The pattern I'm currently wanting to try says the gauge should be 27 sts = 8", 8 rows = 4" in dc. Also, the pattern says the width of the body before sleeves and edging should be 40". I understand what it all means and when I make a swatch of 27 sts/8 rows it comes out right. Then I try making the 135 sts for the full row and it equals 40" wide like it should. But, by the time I get a few rows up the project gets slowly wider and wider. Even though each row still has the correct amount of sts. I'm doing my best to keep my tension the same throughout. How do I make it all come together properly? It surely shouldn't be this complicated to do. Thanks!