Tyrion is all like "Hey remember you were all psyched about me saving the city and said that I was awesome and stuff, did you forget that?"
And Lord Varys is all like "Unfortunately I don't forget anything, guy."
WHAT KIND OF ANSWER IS THAT???
Operates on a few levels.
First, unfortunately for Tyrion, Varys didn't forget the bad stuff Tyrion said about Joffrey.
Second, also unfortunately for Tyrion, Varys doesn't forget all he's learned about "playing the game" and his overall awareness of everything that's going on everywhere in the GoT world. Which means that he knows this is the play he needs to make at this point, for his own purposes. Also, he'd like to be able to forget some of the screwed-up crap that he's aware of, but that's kinda his line of work now, so.
Third, unfortunately for Varys, he of course remembers what he told Tyrion and knows he's screwing him over and feels kinda bad about it, and also knows he won't be able to forget feeling bad about it. (But still, better to be able to feel bad about that than not being able to feel anything because your head is no longer attached to the rest of your body. YOLO.)