The first one is a common tactic for stage demos. This is so that the gameplay doesn't get interrupted by death or anything like that. God mode is on for quite a lot of stage demos, actually.
The second one is apparently intentional just for the show floor. The zombies were made weaker so that the people who were playing the game at the event didn't die all that often.
OT: I was initially interested, at least until the guy started running from the zombies. The size of the horde sure was impressive, and they sprint like crazy, but it just didn't look at that interesting, at least when 50% of the demo was run run run run run, turn around, shoot shoot shoot shoot shoot, run run run run run. The shooting didn't look all that meaty, either.