Have you ever worked a gauge swatch only to find that the stitch count is correct, but the rows are too short or too high? Why is that? Shouldn't you be able to make both stitch and row gauge with the same hook size? The answer lies in the Golden Loop. You see, while hook size determines stitch gauge, the Golden Loop is responsible for row gauge.
The two swatches below were both made with the same size hook for the same number of rows, but by adjusting the Golden Loop, we were able to make one swatch taller than the other.
Okay, so what exactly is the Golden Loop? In any crochet stitch, it's the first loop pulled up after inserting the hook in the stitch. To demonstrate on a double crochet, you would yarn over, insert the hook in the stitch to be worked, pull a loop through, then STOP! The Golden Loop is the loop indicated in the photo below.
Tight crocheters will have a tendency to make this loop shorter, while loose crocheters will make it taller. This is why a given crocheter can meet stitch gauge with a certain hook size but still miss row gauge. So when you are making your gauge swatch, first concentrate on meeting stitch gauge. After that, if your row gauge needs to be adjusted, simply make the Golden Loop taller or shorter and you'll be good to go.