Well I don't know about IMDB. But the other trackers are still saying that the current 2nd week numbers are estimates and not actuals. The actuals usually get released a bit later.
For example. http://boxofficemojo.com/weekend/chart/ Notice how it says studio estimates? The original estimates at box office mojo for the first week of AVATAR were a couple of million lower then the final number (which was $77,025,481 as reported by both box office mojo and the-numbers.com) so this can make a difference.
I don't know how accurate IMDB is either. Since I don't follow it for box office tracking? Does anyone know? Since those numbers you've provided seem a tad lower then several other tracking sites? I don't know which sites considered the most accurate to be honest.