Hello Ms Robocat!
It may not be so much about the terms of the lease as it is the building. In South Beach and SOMA, for example, buildings like 201 Harrison or Cubix have some tiny little living spaces that will naturally rent for far less than a larger, more traditional loft space or condo. That's my best theory. If you think its something else, let me know and we can try to figure it out together.
I frequently find the opposite to be true. I'd be interested in knowing where you're seeing these listings. Most people charge more rent for shorter term leases, to make up for the vacancy factor. I just did a quick Craigslist search and couldn't find anything in the newer buildings/condos with that much square footage for anywhere near that price. A few places were rooms for rent within larger condos, and there were a few that posted $1600/week. I'd love to talk to the people who are renting out at those rates, who knows what they're thinking.
Well a few of the places I looked at with unexpectedly lower rents were in Tendernob, SOMA, South Beach, and were all in the 1000+ sq ft range. And they were in newer buildings/condos. Price range was $1650-2100, which seems really cheap to me. I'm just wondering what the catch is and if they will jack up the rent once I'm in there.