My theory is that he absolutely remembered that he met with russians but perhaps his ethics lawyer...gorelick. .told him he didnt need to report it.
Once top shelf legal shark was hired...voila...he suddenly remembers once legal team found them
Close, but you need to tweak that theory. Kushner did not need to report the meeting. The SF-86 asks for "close or continuing" contacts. The single meeting definitely did not qualify for reporting. But Gorelick could have told him to report it to add ammo to the media collusion story. She would pretend it was her lawyerly duty, but in reality she is not on our side.