Here's what I believe: when you are knitting a project with a specific goal in mind—a deadline, say, or just really really wanting to finish and get it off the needles and into its bath and out the door—something weird happens to time.
Weird, such that you can knit for hours and hours and then discover that you've only moved a couple of inches, when you know darn well that knitting at x pace over y hours should yield a certain number of stitches, and the amount you see on your needles falls far short of that number. I mean, if you count the number of stitches per row and multiply by the number of rows you've completed, you should theoretically come up with a number that approximates x times y. But somehow, a largish proportion of the stitches you know you knit have been sucked off your needles, and you're nowhere near where you thought you should be. It's very perplexing.
This stitch deficit isn't caused by the act of knitting. I know this because I've knit many, many items that have pretty much flown off the needles. It's only when I'm knitting toward a goal that the stitches get sucked away.
So here's my theory: knitting with a goal in mind causes a warp in the time/space continuum. You can think you've been knitting for three hours, and everyone around you will corroborate that yes, they actually witnessed your needles flying, your fingers doing their little knitting dance...but then time folds in on itself and it turns out that you were really only knitting for 45 minutes, tops.
It's definitely caused by the whole goal-oriented knitting thing, and I think if we all went into our projects armed with this knowledge, we'd be a lot healthier and happier. Alternatively, we could simply decided to knit only for pleasure, at which point time straightens itself out again and order is restored to the universe.