One saving grace of the law is that in years when nobody wants to build, the city "banks" the excess for years when construction is over the limit. But we've just about exhausted those savings, and are now hitting the cap.
The developers and the owners of existing office space know that the carryover is about to run dry, and that the cap is going to be in full effect every year for many years to come, and are pricing their long-term leases accordingly. And companies, knowing the same thing, are signing them.
Did you hear that the Sierra Club is going to have to leave the San Francisco headquarters they've occupied since the 1800s, because the rent is now too high for them to afford? (http://www.sfexaminer.com/sierra-club-moving-headquarters-fr...) The irony is that those rising rents are due in no small part to their acting against their stated mission when it comes to allowing tall buildings in this city. They'd rather sprawl people out into the suburbs and beyond, where they'll bulldoze swaths of nature to build their buildings, and the people living and working there will have much larger carbon footprints.