Outside of people's perceptions/memories not necessarily reflecting reality, there is another angle to this: Pierce DOES have to do more this year than he ever had to last year.
Without KG, the Celtics' offense requires a lot more individual effort and is built a lot more on individual brilliance. The good news is that the Celtics have three of the best individual offense producers around in Pierce, Allen, and Rondo, which means that when they are on they can make the offense just as potent if not more-so in the absence of KG. The bad news is that 1) this type of offense requires much more energy to sustain and 2) this type of offense is reliant upon individuals being hot, which makes it less reliable.
On-topic, Pierce is having to put a lot more effort into his offense than he normally does. He also has to put more effort in on defense without KG anchoring the unit. He also has to play much bigger minutes because the Cs need him to be competitive, and the minutes are more intense because the games are closer without KG. Put this all together, and you get a guy putting up bigger numbers than he did in the early part of the '08 playoffs but a guy that also tires during games because he's having to work a lot harder than he has had to for the last 2 years.