Dude can't even remember what he had for breakfast this morning (so he says). Turns out, he does remember what he had, maybe even that random robin that landed on the window frame. Why does he do this?
He avoids responsebility
Indifferent to it
Not a talker/Very reserved
Likes to appear unattentive/Trickster of sorts
IS actually forgetful, but likes to Crank It Up To Eleven for their own reasons
An exaggerated example is that this person pretends to keep forgetting their best-friend-of-several-year's name, when in reality, he knows their full name, family, several lifestyle habits, and a few embarrasing moments.
This person likes to be the Absent-Minded Professor, but again is only faking it (or exagerating it).
They may combine it with Obfuscating Stupidity or Insanity.