The fact that "anatomical bilateral symmetry is a common trait," is a fact, as opposed to a Sufficient and Necessary Statement.
Our principle rule deals with not conferring advantages and being a common trait. So, information about anatomical bilateral symmetry is merely information that something exists, i.e. that anatomical bilateral symmetry is a common trait.
Thus, the first sentence is not giving us another principle rule more so than it is merely giving us information about anatomical bilateral symmetry.
The statement of facts is merely using the first sentence to tell us that something exists, rather than giving us a rule.
Hope that clears things up! Please let us know if you have any other questions.
