The game was being broadcasted live via ESPN, and the announcers (Seirawan, Ashley) were saying that X3D Fritz was lost and did not understand how to play the position, which was obvious to everyone. Usually the Fritz team would resign at a score of 2.5 or 3.00. However with network broadcasting, they wanted to end the embarrassment a little earlier. Check out GM Boris Alterman's commentary on game three. Boris at one time was a 2nd of Kasparov.
links to www.chess.fm
A good example of early resignation was during Kramnik - Deep Fritz match in Bahrain in 2002.
Sometimes chess games end to early for common folks. I recall Game 2 Kramnik - Deep Fritz , where Deep Fritz resigned in a far from clear endgame. It was a win for Kramnik, but it was a win quite a few moves in the future It ended a bit early even for GM Nigel Short. Short challenged Kasparov in 1993 for the World Chess Championship. It was an exciting match, Nigel had his chances early, but he came up short.
GM commentator Nigel Short was critical of the Fritz
Team's decision to resign. "At least for the thousands of chess fans out
there who will see this game and not understand why black resigned," he
said. "There are several long variations for white to calculate, many
choices to make, it's not that simple. Black should have played on for at least
10 more moves."