OK, so I've read the WHO documents and looked at some of the research. From everything I have read the 6 month date is misleading. Even the WHO state that some babies are ready at 4 months and can be weaned then, in the same document that says 6 months is recommended.
SO, wouldn't it have been better if they had said:
"When your baby can sit up unaided, is interested in food and has lost the tongue thrust reflex then they are ready to try solids. If they reject it, try again later"
Surely that would be much simpler and would be much more useful to parents?