Why when I wear a certain dressing gown does my cat?

Tiger jump up on my lap and start kneading my shoulder with her front paws and appears to be sucking my dressing gown. It's always in the same place and she doesn't do it with any other garment. In fact it's the only time she gets on my lap. I thought in the beginning that there was something there - like a stain etc, but I have washed it many times but to no avail.