There is no doubt that Avatar 2 is having a great run
I think the main problem is having huge huge expectations before release.
I remember after AEG success many people putting expectations over 3 billions.
According to BOM, the biggest movie not to cross 1 billion is harry potter and the deathly hallows part 1 at 977 million.
JWD will mostly cross this number, so it may be the biggest movie missing 1 billion.