Maybe because the scientific advice may be ambiguous and leave room for interpretation which the politicians must exercise under the best of circumstances.
My reading of the letter is that it challenges the scientific advice, as it has been communicated in the media, and seeks to clarify the "wiggle space" politicians have to work with.
My reading of the letter is that it challenges the scientific advice, as it has been communicated in the media, and seeks to clarify the "wiggle space" politicians have to work with.