I could have thought about the personal tension as well....
It's obvious that if you're a tight knitter, you may knit on bigger needles than suggested, and the contrary is true for a loose knitter.
I'm one of the latter, and I bought some yarn, it says knit on 10-12 mm, and I'll knit it on 8mm, to get a good result.
The ways of Yarn are inscrutable.
I'm NOT a "miss", I'm a lady!