EDIT: I should add there is likely to still be plenty of debate on the shape of the slope post peak.

The production curves of individual fields and regions may tend towards being more or less bell-shaped (is that true?), but I can see a reason why the world curve would have a sizable plateau.

Because there's a global market in oil, there's no immediate crisis for those large consumers that can buy or otherwise extract the oil from the global market. But matters are quite different in the case of the global peak. Here, given the way things are, there's bound to be a desperate attempt to ward off decline. And it can no doubt be done for a while.

Of course that flattening, a long plateau, means the dropoff is sharper when it does arrive.

Pure speculation -- caveat emptor.