Keep in mind that bamboo usually is not the natural bamboo fiber - that would not be soft. Bamboo actually is a highly processed fiber. In fact the Federal Trade Commission now states bamboo should be called "rayon made from bamboo" since it is essentially the same process as rayon made from cotton or other cellulose plant scrap. Thus, bamboo batting is actually rayon batting made from bamboo - thus the softness.
Here is a link
http://www.ftc.gov/opa/2010/02/bamboo.shtm