The numbers are not surprising when its a new franchise vs an established franchise. Nor a surprise when Transformers was released on 4000 screens in the U.S. and piecemeal across the world (with major markets to go) and Harry Potter was released on on 9000 screens in the U.S. and worldwide in all markets.
While I think the 9000 screens is a bit excessive, Transformers decisions makers should note the success of the simultaneous worldwide release and copy it for Transformers II. Really though, considering the number of screens and the history of the Potter films, Transformers more then held its own. Usually in the face of such competition, everyone else gets lost in the crowd but instead had a weekend that most movies brag about. It should also be noted it "only" dropped 49% from the previous weekend. Considering the average film looses about 60-65% percent from one weekend to the next, that's a pretty darn good number. It looks like the race for summer box office champ is still undecided. Harry 5 is the last major film of this class for the summer. How much it drops next weekend might give a clearer picture of who ends up on top. I just want to hear the official news of a sequel.