The only scenario I can see where this would make sense is if SIAI expects small donors to donate less than $(1/2)N in a dollar-for-dollar scheme, so that its total gain from the fundraiser would be below $(3/2)N, but expects to get the full $(3/2)N in a two-dollars-for-every-dollar scheme. But not only does this seem like a very unlikely story [...]

One year later, the roaring success of MIRI's Winter 2013 Matching Challenge, which is offering 3:1 matching for new large donors (people donating >= $5K who have donated less that $5K in total in the past) -- almost $232K out of the $250K maximum donated by the time of writing, with more than three weeks time left, where the Winter 2012 Fundraiser the parent is commenting on only reached its goal of $115K after a deadline extension, and the Summer 2013 Matching Challenge only reached its $200K goal around the time of the deadline -- means that I pretty much need to eat my hat on the "very unlikely story" comment above. (There's clearly an upward growth curve as well, but it does seem clear that lots of people wanted to take advantage of the 3:1.)

So far I still stand by the rest of the comment, though:

[...] even if it did happen it seems that you should want to donate in the current fundraiser if you're willing to do so [at 1:1 matching], since this means that more matching funds would be available in the later two-dollars-for-every-dollar fundraiser for getting the other people to donate who we are postulating aren't willing to donate at dollar-for-dollar.