The first avengers movie had less competition and was less frontloaded than Infinity War will end up being. IW is a sequel and is burning up demand faster.
You do realize that IW's opening weekend was $55M higher than Black Panther's OW, but IW's 2nd weekend #s that just came in are only 900k bigger than BP's 2nd weekend, right? IW will start losing in daily numbers to Black Panther today, and the later will eventually close the gap. This movie is definitely not gonna be " easily over 700M ".
EDIT: It actually started losing to Black Panther 2 days ago. BP's 2nd Saturday and Sunday were bigger. IW only had Friday over BP. If this movie hits $700M, which I have no reason to believe it will, it's gonna be a loooong stretch. Not gonna be easy at all.
Keep in mind that the numbers we have are just estimates. Ones that Disney/Marvel puts out mid-Sun. There's a good shot that they have underestimated the weekend. We'll have to see when actuals come out later today.
Lol, I've been closely watching the box office for 15 years. I am aware they are estimates, but Sunday numbers are the easiest to predict and thus the weekend "estimates" are always pretty close to actuals. Don't expect much of a difference when actuals kick in.