NATS have nothing to do with the medical. CAA will inform them you have passed, and from now on it's your responsibility to ensure you renew it and inform the CAA Medical Section if you have any doubts as to its validity.
On other other question, yes because of the workload perhaps, but also because depending on which course you get selected for, the dates are different (without even considering the possiblity of having to resit exams/courses etc).