14 Reasons Your Gauge Varies

1. Different needles can change how you knit. The materials they are made from affect "slipablity", the point shape can change how you form your stitches and a 4mm in one manufactures needle may not be exactly the same size as one from a different manufacturer.
2. Changing from straight to circular needles can change your gauge as you often hold the needles in a slightly different way.
3. Knitting in the round vs. knitting flat can change your gauge especially in stocking stitch because there are no purl stitches.
4. Your mood changed because you had a fight with your mother...sister or boyfriend.
5. You drank a glass of wine or a cocktail.
6. You are knitting in a different technique and you tense up slightly until you become more practiced and comfortable at it.
7. Some Knitters make their purl stitches more loosely so a stitch pattern with a lot of purls may need to be worked on smaller needles.
8. All yarns are not created in the same spinning method even if they are the same weight.So for example one DK yarn may need to worked on a smaller or larger needle to get the correct gauge as compared to another.
9. Fiber content variations can make you knit a silk vs. wool yarn of the same weight at different gauges.
10. A large number of stitches on a project can change your gauge from what you got on a  4" swatch.
11. Knitting becomes more even as it is washed and worn so gauge may change from the original swatch.
12. The weather can affect gauge, hot humid weather can make the yarn or needles feel different and your hands may perspire or become oily which can impact the manner in which you tension your yarn.
13. Changing needles sizes can sometimes affect row gauge more than stitch gauge initially and that difference is often ignored by Knitters on the swatch but has consequences on the project in that the knitting shifts as it becomes more even with time, wear and washing.
14. Spreading your work too far apart along the needles can also cause variations in gauge by creating irregular sized stitches. 

Remember that you can get gauge in an yarn weight that is inappropriate to your project and create a project that is too stiff for comfort or too loose to stand up to normal wear. Always consider the fabric that you are creating and it's appropriateness for your project.