I'm not really surprised to be honest. Not much was going on this weekend afterall. That said, while I felt Godzilla could have been better, but also think it was really awesome. The scenes with the monsters were just incredible. It suffers from an average story, Aaron Taylor-Johnson's part was just not written well and managed to be the most boring part of the film. The best actor, in both my opinion and that of my girlfriend (who is getting a Masters in Theatre), was Elizabeth Olson. She did really really well in her part and pulled off some pretty darn tough scenes like a champ. I feel that she was largely ignored because she played second fiddle to a really boring lead character.
That said: I wonder if people were expecting Oscar caliber anything in the film. It was a mistake to not concentrate more on the Muto/Godzilla aspect of the movie, no doubt. But the human aspect of Godzilla movies has always been extremely boring. I feel like they made the same poor decision with human aspect that they did with the Transformers films. IIRC, they thought that no one would want to watch a movie just about Transformers and not have a human character for the audience to associate with. That is, by and large, the main complaint in the transformers films and the complaint here again, even from major film critics. Maybe that will start to change with reviews basically hating the human story while simultaneously applauding everything that involved the monsters.
Also, Neighbors was a damn funny movie. It deserves the spot it currently holds, IMO. I just think it's funny when people are like "it looked like trash". You have little to say of its spot in the box office when you did not even bother seeing it. It wasn't perfect, but as someone who loves comics, it deserved to knock ASM2 out of first place. And Godzilla definitely has earned it's number one spot based on what is currently out there.