Obama speaks to a campaign rally of 18,000 people, in an area that accommodates only 5,000. It turns out that the media uncritically repeats any count the Obama campaign gives them, without doing even the modicum of fact-checking that would show that the number is impossible.
This is a truly nasty pattern. Anything a Republican says is aggressively “fact-checked” (and usually ruled false even though it was true), while anything from Obama is accepted at face value. This has been going on for a long time.