What Happened to Smyslov?

Dear readers, I have a few books by Efim Geller, and they are all excellent. Starting with his books on the King's Indian Defense, and followed by his classic, "The Application of Chess Theory", his games emanate great power!

In the following game, one of his immortal ones, he sacrifices his Queen three times! The first two times Smyslov cannot take it; the third time brings immediate resignation by Smyslov. This game, in which Smyslov plays the Grunfeld, one of his favorite defenses, features and attack by Geller, so powerful, that Smyslov is lost right out of the opening. It was played in a Candidate's Match in 1965. So far, nothing surprising here!

What IS surprising is that, in 1959, Smyslov achieved a wonderful victory with the Black pieces in this same kind of setup....but he played it correctly in 1959, only to forget to play the main antidote to White's attack in 1965.

What happened to Smyslov?

I hope that someone can answer this question because it is such an important variation in the main lines of the Grunfeld, I do not see why Smyslov would avoid playing the correct plan with Black!

But let us go to the games, and forgive my ignorance, because the Grunfeld is one of those openings for Black which I admire from a distance; in modern times perhaps Grischuk and MVL are the greatest exponents of the Black side. It always looks like the kind of position which I would feel lost in; it probably does not match my style (not that I have much of a style, mind you).

Here is the game from 1965:

Geller writes:

"  INVULNERABLE QUEEN

The variation selected by Smyslov did not take me unawares. When preparing for the match, my coach Semyon Furman and I correctly supposed that if the score was in my favour and the ex-World Champion had to go in for sharp play as Black, he might have recourse to this complicated line. And so it came about. Although the variation had been seen in practice often enough, I succeeded in turning its negative aspects into account."

Reference game: Tukmakov-Stein, Alekhine Memorial 1971:

Now here is Smyslov beating Gligoric with this same system, in 1959! Take a look!

Now, the question is: If Smyslov knew the proper plan (and I have to assume he knew, because he beat Gligoric with it in 1959), how come he did not apply it in 1965? Did he really forget? I mean, this was a Candidates' Match! How can you forget?

Reference game: Spassky-Fischer, Siegen Olympiad 1970: