I don't know how much this is worth but right after Krugman won the Nobel prize, the econ dept said they would use this opportunity to expand the department, so this might be why they have so many admits this year.
Also, based on what I can see on TM, the people who got into Princeton usually got into a bunch more top 10 schools as well. Choosing between the top 10 is sometimes is a little arbitrary, so Princeton has to expect a fair amount to decline.