[Boy, their definition of a "memo" is sure different than mine. Used to be something your secretary hands you about a missed call.]
IIRC, every FBI agent has a law degree.  This means they went through law school and use lawyer's standards.  A memo for lawyers contains all kinds of information from, as your example provided a missed call, to a case or argument to be presented in court.  It is common for memos to contain work product such as Comey's memos are alleged to contain.