"Don't chase the wheel," said My Teacher Paula.
The same applies to knitting. Rushing fiercely into garment production, without having invested time in gauge preparation.
Who among us, eager to begin that journey of a million stitches, has not bypassed knitting our own test swatch? The designer made that decision for us, we rationalize.
Yet, though the yarn be the same and the needle size identical, the designer's knitting is not yours, any more than their handwriting is. Glenna's knitting always looked done from handspun; Ivy's mimics a machine's regularity.
Over the course of a million, or a thousand, or a hundred, or a dozen stitches, those differences add up.
You're even different from yourself, over the course of a large test swatch. I always cast on double or three times what the gauge measurement would predict. If the gauge is 19 stitches to 4 inches, I'll cast on about 42–45. Then I'll measure 4 inches worth at three or four spots along that length, settling on the average of those measurements.
I know, I know it's not micrometer material, but when it comes to handwork, numbers are not as cut-and-dried as you might imagine.
And what you begin, will inexorably reveal the mathematics of its foundation. The implications of an idea.
Be sound in your foundations. Knit that test swatch!