The difference between 5 Seconds of Summer's聽CALM聽and The Weeknd's聽After Hours聽is 2,556 units sold.
Due to an error in shipping in the US 10,000 copies of Calm were released early. This meant that they did not count towards the total, losing them their 4th number 1 which they would have got without the error.
This petition I found through other fans is being created to change their position on the Billboard 200 so that it reflects the 10,000 copies that were released early, which they have done for countless artists in the past but somehow ignore 5sos.
According to some fans Billboard has blocked fans on Twitter for raising this and that a crew member tweeted that the petitions seems to be working.