This could be construed to state that the prize was awarded for "what he accomplished" in calendar year 2008, rather the lengthy period between his inauguration and February 1, 2009.
Not just construed — that’s what it was supposed to be, and has generally BEEN. It’s why the nominations are done by February 1st, to give people up to 1 month after the end of the year to get in their nominations, so the committee has a 8 months to evaluate what all the nominees did in the previous year.
Obama seems to be the first person to get picked solely for what they “did” AFTER they were nominated.
Which means that, if Obama had acheived World Peace last week, he wouldn’t get the nomination for next year, since they already GAVE the award for 2009 to Obama.