What is the meaning of Carl Rogers' thirteenth proposition in layman's terms?

Behaviour may, in some instances, be brought about by organic experiences and needs which have not been symbolised. Such behaviour may be inconsistent with the structure of the self, but in such instances the behaviour is not "owned" by the individual.

I don't blame you for asking! This is referring to the fact that each person, or individual, has an internal "self" based on their experiences with the outside world (which is based on the senses and information gotten while interacting with the world). As the individual has different experiences, each one is filed away in the mind - either accepted and used as a symbol for some larger set of experiences, which is then filed away based on relation to the "self"; or rejected by the individual as not belonging to the "self;" or ignored if the individual sees no value in the experience. Sometimes, experiences and needs might not be consciously brought into the mind and made a part of the "self," but will still have an effect on the individual's behavior. This behavior might then seem to be at odds with the rest of the individual's identity or "self," however, the actual behavior is not a true part of the "self," but is adopted by the individual.