Thread:Elledy92/@comment-26838855-20190425183005/@comment-1894786-20190502012521

WatchMojo just released this video where they attempt to explain the time travel in the movie, and lay out two good points for the whole Captain America thing; Steve takes more Pym Particle vials than they needed to return to the present (one vial is good for two round-trip jumps, according to Scott's dialogue before Clint volunteers), they only needed two but Steve takes four.

And they don't necessarily need to appear on the machine, much like how Tony and Steve weren't on the machine in New York when they went to New Jersey. However, it can be assumed that the machine needs to be on during the time they make the jump (which it was in 2023).

This backs up my theory that Steve appears on the bench when Bruce registers him traveling but the machine doesn't fully activate.