What are the "OK games"? The 6 goals on 37 shots was a very bad game and then the other two .880 games were just bad games. I realize the award is for the whole post-season, but as hawksrule pointed out, he went from basically invincible in games 1-14 of the run, to very ordinary in his final 7 games, all in the Final.
What helped Giguere win the award too, was that game 7 was close at the time the ballots needed to be in (ballots need to be submitted no later than with 10 minutes to go in the deciding game). So, if the Ducks had mounted a 2-goal comeback and won the game, then Giguere would have been the clear choice to win the award. That's also why you rarely see a losing player win. Most Finals don't end in 7 games. The team that wins is generally made more clear and then the writers just pick players from the team that's going to win.