Comey is either Psychic, or he knew Sessions would recuse himself by their design.
Or he lied and didn’t tell Sessions any of this because there was nothing to tell him but since he’s now making this stuff up from scratch he needs to explain why his boss never heard of it. So he makes up some stuff about assuming Sessions would recuse himself so he decided to just keep him in the dark and go rogue.
Or the memo was written much later....
..either that or Comey was talking to those ‘advisors’ who convinced Sessions to recuse himself. Why did Sessions listen to those advisors? I HAVE NO IDEA.