Looks like it's an old survey, maybe they've had a wash or two since then :)
I'm the same as coccinelle, I have a top sheet because battling with a duvet cover's a bit much every week.
Yes, robinia. It does look like an old survey. It's written about in this coming November's Prima magazine. They are obviously behind the times.
It is a good article though telling you how often pillows and duvets should be washed and what to do with your matress.