It might not say that exact combo on the yarn band–that’s the gauge the designer or test knitters got, and the gauge that will give you the same look as the pattern. You need to find YOUR gauge for the project.
Look for a yarn that says 20-22 sts, get out your 3.75 needles and cast on 31-33 stitches. Work enough rows for it to be square, and then measure the middle 4". If you have fewer than 21 stitches in 4", try a smaller needle. If you have more, try a larger needle. Row gauge is often less important than stitch gauge.
You often don’t have to work the whole swatch–if 33 stitches isn’t working up to be about 6", you can stop and change needles sooner, but you do need to do several rows to see how it spreads out once it’s away from the needles.