Also, a lot of us have a tendency to knit our gauge swatches very carefully, which often means more tightly than we normally knit. Then, when we get into the actual knitting, we relax and the knitting loosens up. If this happens to you consistently, it would be helpful to measure your work after 2 inches and every couple of inches thereafter. If you notice it spreading out, switch to a smaller needle.
Remember: you're the boss of your knitting and you can make it do anything you want.