Well, there was apparently concern that if the ceremony went past its appointed finishing time of between midnight and 12:30 a.m., it would make it difficult for some of the 80,000 expected spectators to get home via public transportation.
Therefore several scenes have been shortened, while one involving stunt bikes has been eliminated altogether. The changes reportedly amount to about a half an hour.
"Performers are rehearsing sections and transitions to ensure they are as tight as possible," a London 2012 spokesperson said in a statement. "This is normal in any production whether it be theatre or ceremonies."